runtime: String.IndexOfAny is slower than hand-written naive implementation on x86

See: https://github.com/NuGet/NuGet.Client/pull/1590#discussion_r130262373 and https://github.com/aspnet/Mvc/issues/5362.

On short strings and only two char arrays, String.IndexOfAny is significantly slower than a hard-coded version, and on x86 (non-RyuJIT) even slower than a hand-written naive implementation where there is no matches.

Method	Job	Jit	Platform	Input	Mean	Error	StdDev	Scaled	ScaledSD	Faster?
IndexOfAny_Baseline	LegacyJitX64	LegacyJit	X64	This is a short string.	3,690.3 us	43.6378 us	36.4396 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX64	LegacyJit	X64	This is a short string.	2,276.0 us	27.5134 us	25.7360 us	0.62	0.01	✓
IndexOfAny_HandWrittenLoop	LegacyJitX64	LegacyJit	X64	This is a short string.	6,550.1 us	123.8535 us	96.6966 us	1.78	0.03
IndexOfAny_Baseline	LegacyJitX86	LegacyJit	X86	This is a short string.	6,552.9 us	65.9870 us	61.7243 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX86	LegacyJit	X86	This is a short string.	2,314.6 us	48.8886 us	71.6603 us	0.35	0.01	✓
IndexOfAny_HandWrittenLoop	LegacyJitX86	LegacyJit	X86	This is a short string.	5,143.8 us	51.0904 us	47.7900 us	0.79	0.01	✓
IndexOfAny_Baseline	RyuJitX64	RyuJit	X64	This is a short string.	3,666.7 us	61.5387 us	57.5634 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	RyuJitX64	RyuJit	X64	This is a short string.	2,400.2 us	22.9174 us	21.4370 us	0.65	0.01	✓
IndexOfAny_HandWrittenLoop	RyuJitX64	RyuJit	X64	This is a short string.	6,126.5 us	118.1990 us	116.0872 us	1.67	0.04
IndexOfAny_Baseline	LegacyJitX64	LegacyJit	X64	/This is a short string.	1,196.7 us	8.0287 us	7.1172 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX64	LegacyJit	X64	/This is a short string.	206.3 us	1.3898 us	1.3000 us	0.17	0.00	✓
IndexOfAny_HandWrittenLoop	LegacyJitX64	LegacyJit	X64	/This is a short string.	464.2 us	8.2176 us	7.2847 us	0.39	0.01	✓
IndexOfAny_Baseline	LegacyJitX86	LegacyJit	X86	/This is a short string.	1,668.2 us	11.5627 us	10.8158 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX86	LegacyJit	X86	/This is a short string.	233.1 us	0.7199 us	0.6734 us	0.14	0.00	✓
IndexOfAny_HandWrittenLoop	LegacyJitX86	LegacyJit	X86	/This is a short string.	350.4 us	1.5306 us	1.3568 us	0.21	0.00	✓
IndexOfAny_Baseline	RyuJitX64	RyuJit	X64	/This is a short string.	1,344.7 us	35.3044 us	49.4918 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	RyuJitX64	RyuJit	X64	/This is a short string.	236.6 us	1.6766 us	1.4001 us	0.18	0.01	✓
IndexOfAny_HandWrittenLoop	RyuJitX64	RyuJit	X64	/This is a short string.	384.6 us	5.8540 us	5.1894 us	0.29	0.01	✓
IndexOfAny_Baseline	LegacyJitX64	LegacyJit	X64	This is a/short string.	2,074.6 us	14.7860 us	13.1074 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX64	LegacyJit	X64	This is a/short string.	920.1 us	14.5393 us	12.1410 us	0.44	0.01	✓
IndexOfAny_HandWrittenLoop	LegacyJitX64	LegacyJit	X64	This is a/short string.	3,059.0 us	26.8631 us	20.9729 us	1.47	0.01
IndexOfAny_Baseline	LegacyJitX86	LegacyJit	X86	This is a/short string.	3,821.6 us	22.9191 us	20.3171 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	LegacyJitX86	LegacyJit	X86	This is a/short string.	1,256.0 us	7.6582 us	7.1635 us	0.33	0.00	✓
IndexOfAny_HandWrittenLoop	LegacyJitX86	LegacyJit	X86	This is a/short string.	3,824.8 us	36.8087 us	34.4309 us	1.00	0.01
IndexOfAny_Baseline	RyuJitX64	RyuJit	X64	This is a/short string.	2,096.2 us	15.8659 us	14.8410 us	1.00	0.00
IndexOfAny_HandWrittenLoop_HardcodedChars	RyuJitX64	RyuJit	X64	This is a/short string.	1,045.3 us	12.6123 us	11.7975 us	0.50	0.01	✓
IndexOfAny_HandWrittenLoop	RyuJitX64	RyuJit	X64	This is a/short string.	3,116.7 us	62.2240 us	103.9623 us	1.49	0.05

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static int IndexOfAny(string value, char[] array)
    {
        return value.IndexOfAny(array);
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static int IndexOfAny_HandWrittenLoop_HardcodedChars(string value)
    {
        for (int i = 0; i < value.Length; i++)
        {
            char c = value[i];

            if (c == '/' || c == '\\')
            {
                return i;
            }
        }

        return -1;
    }

  [MethodImpl(MethodImplOptions.NoInlining)]
    public static int IndexOfAny_HandWrittenLoop(string value, char[] array)
    {
        if (array.Length == 0)
            return 0;

        for (int i = 0; i < value.Length; i++)
        {
            char c = value[i];

            foreach (var ch in array)
            {
                if (c == ch)
                    return i;
            }
        }

        return -1;
    }

Benchmark is here (using BenchmarkDotNet): https://gist.github.com/davkean/dab5c068e3f907709f57a757a7e7fb3a.

@benaadams Seems to think that the code creates a probabilistic map is over kill for a two char array.

Updated: Add a few more test cases, and updated handwritten loop to handle empty chars.