Recently atomicrmw started to support fadd/fsub:
https://reviews.llvm.org/D53965
However clang atomic builtins fetch add/sub still does not support
emitting atomicrmw fadd/fsub.
This patch adds that.
Reviewed by: John McCall, Artem Belevich, Matt Arsenault, JF Bastien,
James Y Knight, Louis Dionne, Olivier Giroux
Differential Revision: https://reviews.llvm.org/D71726
Yaxun (Sam) Liu