[clang] [HLSL] Invert the result of `firstbithigh` (PR #166419)

[Ashley Coleman via cfe-commits]

V-FEXrt wrote:

Here is some further reading/context that we want to make sure we've applied here https://github.com/Microsoft/DirectXShaderCompiler/issues/169

https://github.com/llvm/llvm-project/pull/166419