[clang] [llvm] [HLSL] Implement elementwise firstbitlow builtin (PR #116858)

[Ashley Coleman via llvm-commits]

V-FEXrt wrote:

Yeah I figured they weren't broken by me but I had assumed I needed all green before merging so was waiting for the fix to make it into main but happy to merge as is if that's acceptable

https://github.com/llvm/llvm-project/pull/116858