[AMDGPU] Allow w64 ballot to be used on w32 targets (#80183)

Summary:
Currently we cannot compile `__builtin_amdgcn_ballot_w64` on non-wave64
targets even though it is valid. This is relevant for making library
code that can handle both without needing to check the wavefront size.
This patch relaxes the semantic check for w64 so it can be used
normally.