Better test narrowing

Better test narrowing

The previous implementation attemped to narrow down the test mask to have the same type as the memory operand. Appart from unnecessary complications that arise from sign extension the approach doesn't yield the best results in terms of code size.

FX diff shows a 586 bytes improvement without any regressions. All improvements are of the form

test     byte  ptr [rcx+64], 1

instead of the 3 byte longer

test     dword ptr [rcx+64], 1

Commit migrated from https://github.com/dotnet/coreclr/commit/07d7c735947a7bc11a1e168fdde1388cc66f0b78