runtime: RyuJit: Functions with stackalloc won't inline

Functions using stackalloc will fail to inline with reason unknown opcode

Discovered this as part of this PR https://github.com/davidfowl/Channels/pull/145 (should have probably raised it then)

Example function that will fail to inline even with aggressive inlinining

public unsafe bool TrySliceTo(byte b1, byte b2, out ReadableBuffer slice, out ReadCursor cursor)
{
   byte* twoBytes = stackalloc byte[2];
   twoBytes[0] = b1;
   twoBytes[1] = b2;
   return TrySliceTo(new Span<byte>(twoBytes, 2), out slice, out cursor);
}

Changed function that will inline successfully without aggressive inlinining

public unsafe bool TrySliceTo(byte b1, byte b2, out ReadableBuffer slice, out ReadCursor cursor)
{
   // use address of ushort rather than stackalloc as the inliner won't inline functions with stackalloc
   ushort twoBytes;
   byte* byteArray = (byte*)&twoBytes;
   byteArray[0] = b1;
   byteArray[1] = b2;
   return TrySliceTo(new Span<byte>(byteArray, 2), out slice, out cursor);
}

/cc @AndyAyersMS @JosephTremoulet category:cq theme:inlining skill-level:expert cost:medium