[clang] [llvm] [HLSL] implement elementwise firstbithigh hlsl builtin (PR #111082)

[Sarah Spall via cfe-commits]

spall wrote:

> It is not correct to limit firstbithigh to 32 bit integers. There are a couple of reasons that might make one think that it is, but we do in fact want/need to support int16 and int64 here.
> 
>     1. The [HLSL docs](https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/firstbithigh) only mention integer. Unfortunately that's true for most of these functions, as many of these docs were not updated as 64 and 16 bit types gained support in the language.
> 
>     2. DXC's SPIR-V implementation doesn't handle 16- and 64-bit for these functions. As noted in [Incorrect SPIR-V generated when `firstbithigh` is used on a `uint64_t` microsoft/DirectXShaderCompiler#4702](https://github.com/microsoft/DirectXShaderCompiler/issues/4702) we decided not to implement those overloads in DXC, but should reconsider this for clang.
> 
> 
> Despite all of this, DXC does indeed support 16- and 64-bit overloads, as seen here: https://hlsl.godbolt.org/z/qbc17xz35
> 
> Note that the return type of the operation is not overloaded - all of the overloads of this function return uint.

Why is the return type not overloaded? It is overloaded for countbits which is a similar function.

https://github.com/llvm/llvm-project/pull/111082