[AMDGPU] Change Clang AMDGCN atomic inc/dec builtins to take unsigned values

builtin_amdgcn_atomic_inc32(uint *Ptr, uint Val, unsigned MemoryOrdering, const char *SyncScope)
builtin_amdgcn_atomic_inc64(uint64_t *Ptr, uint64_t Val, unsigned MemoryOrdering, const char *SyncScope)
builtin_amdgcn_atomic_dec32(uint *Ptr, uint Val, unsigned MemoryOrdering, const char *SyncScope)
builtin_amdgcn_atomic_dec64(uint64_t *Ptr, uint64_t Val, unsigned MemoryOrdering, const char *SyncScope)

As AMDGCN IR instrinsic for atomic inc/dec does unsigned comparison,
these clang builtins should also take unsigned types instead of signed
int types.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D83121