[InstCombine] Extend "idempotent" atomicrmw optimizations to floating point

An idempotent atomicrmw is one that does not change memory in the process of execution.  We have already added handling for the various integer operations; this patch extends the same handling to floating point operations which were recently added to IR.

Note: At the moment, we canonicalize idempotent fsub to fadd when ordering requirements prevent us from using a load.  As discussed in the review, I will be replacing this with canonicalizing both floating point ops to integer ops in the near future.

Differential Revision: https://reviews.llvm.org/D58251

llvm-svn: 355210