Propagate fmf for setcc/select folds

Summary: This change facilitates propagating fmf which was placed on setcc from fcmp through folds with selects so that back ends can model this path for arithmetic folds on selects in SDAG.

Reviewers: qcolombet, spatel

Reviewed By: qcolombet

Subscribers: nemanjai, jsji

Differential Revision: https://reviews.llvm.org/D62552

llvm-svn: 362439