[InstCombine] Propagate fast math flags through selects

[InstCombine] Propagate fast math flags through selects

Summary:
In SimplifySelectsFeedingBinaryOp, propagate fast math flags from the
outer op into both arms of the new select, to take advantage of
simplifications that require fast math flags.

Reviewers: mcberg2017, majnemer, spatel, arsenm, xbolva00

Subscribers: wdng, javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65658

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368175 91177308-0d34-0410-b5e6-96231b3b80d8