The intrinsic @llvm.amdgcn.flat.atomic.{fadd,fmax,fmin} can only be
selected for flat address spaces (constant, flat and global).
This patch restricts the cases over which GCNTTIImpl::rewriteIntrinsicWithAddressSpace
rewrites the intrinsic.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D149938