runtime: Performance regression: WebUtility.UrlDecode 2.5x slower on AMD only

For some reason, the System.Net.Tests.Perf_WebUtility.Decode_DecodingRequired benchmark have regressed 2.5x on AMD only.

Repro

git clone https://github.com/dotnet/performance.git
cd performance
# if you don't have cli installed and want python script to download the latest cli for you
py .\scripts\benchmarks_ci.py -f netcoreapp2.2 netcoreapp3.0 --filter System.Net.Tests.Perf_WebUtility.Decode_DecodingRequired
# if you do
dotnet run -p .\src\benchmarks\micro\MicroBenchmarks.csproj -c Release -f netcoreapp2.2 --runtimes netcoreapp2.2 netcoreapp3.0 --filter System.Net.Tests.Perf_WebUtility.Decode_DecodingRequired

System.Net.Tests.Perf_WebUtility.Decode_DecodingRequired

Conclusion	Base	Diff	Base/Diff	Modality	Operating System	Arch	Processor Name	Base Runtime	Diff Runtime
Same	182.30	182.19	1.00		ubuntu 18.04	64bit	Intel Xeon CPU E5-1650 v4 3.60GHz	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Same	154.89	153.16	1.01		Windows 10.0.18362	64bit	Intel Xeon CPU E5-1650 v4 3.60GHz	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Same	309.98	305.69	1.01		ubuntu 16.04	64bit	Intel Xeon CPU E5-2673 v4 2.30GHz	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-01
Same	320.10	331.77	0.96		ubuntu 18.04	64bit	Intel Xeon CPU E5-2673 v4 2.30GHz	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-01
Same	211.93	210.11	1.01		macOS Mojave 10.14.5	64bit	Intel Core i7-5557U CPU 3.10GHz (Broadwell)	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Same	182.27	180.18	1.01		Windows 10.0.18362	64bit	Intel Core i7-5557U CPU 3.10GHz (Broadwell)	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Faster	138.34	130.48	1.06		Windows 10.0.18362	64bit	Intel Core i7-7700 CPU 3.60GHz (Kaby Lake)	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Slower	175.79	420.39	0.42		Windows 10.0.18362	64bit	AMD Ryzen 7 1800X	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Slower	134.67	327.60	0.41		Windows 10.0.18362	64bit	AMD Ryzen 9 3900X	.NET Core 2.2.6	.NET Core 3.0.0-preview8-28373-12
Same	182.93	173.77	1.05		Windows 10.0.18362	32bit	Intel Xeon CPU E5-1650 v4 3.60GHz	.NET Core 2.2.6	.NET Core 3.0.0-preview8-27919-09
Same	205.06	203.17	1.01		Windows 10.0.18362	32bit	Intel Core i7-5557U CPU 3.10GHz (Broadwell)	.NET Core 2.2.6	.NET Core 3.0.0-preview8-28373-17

/cc @danmosemsft @tannergooding @billwert @DrewScoggins