Guard FMF context by excluding some FP operators from FPMathOperator

Summary:
Some FPMathOperators succeed and the retrieve FMF context when they never have it, we should omit these cases to keep from removing FMF context.

For instance when we visit some FPMathOperator mapped Instructions which never have FMF flags and a Node was associated which does have FMF flags, that Node today will have all its flags cleared via the intersect operation.  With this change, we exclude associating Nodes that never have FPMathOperator status under FMF.

Reviewers: spatel, wristow, arsenm, hfinkel, aemerson

Reviewed By: spatel

Subscribers: llvm-commits, wdng

Differential Revision: https://reviews.llvm.org/D51145

llvm-svn: 342081