[llvm] [NFC][AMDGPU] Assert no bad shift operations will happen (PR #108416)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I am a silly, `Mask` is `int64_t`, and `has_single_bit` only works on unsigned integers. @jplehr Is the procedure to revert this in main and resubmit the fixed version or I just push a follow-up fix? Apologies!

Either way 

https://github.com/llvm/llvm-project/pull/108416