Recently atomicrmw started to support fadd/fsub:
However clang atomic builtins fetch add/sub still does not support emitting atomicrmw fadd/fsub.
This patch adds that.
TargetInfo::isFPAtomicFetchAddSubSupportedByBits is added to condition supporting of fp type
in atomic fetch add/sub builtins. By default it returns false. Target needs to override it
to enable it. This patch only enables it for amdgcn. For other target this patch is NFC.