[InstSimplify] fold select (fcmp X, Y), X, Y

This is NFCI for InstCombine because it calls InstSimplify, 
so I left the tests for this transform there. As noted in
the code comment, we can allow this fold more often by using
FMF and/or value tracking.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346169 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed