[SelectionDAG] Make sure FMF are propagated when getSetcc canonicalizes FP constants to RHS.

getNode handling for ISD:SETCC calls FoldSETCC which can canonicalize
FP constants to the RHS. When this happens we should create the node
with the FMF that was requested. By using FlagInserter when can ensure
any calls to getNode/getSetcc during canonicalization will also get the flags.

Differential Revision: https://reviews.llvm.org/D88063