[SDAG] Add AllowContract to SNodeFlags

Properly propagate the FMF from the LLVM IR to this flag.

This is toward moving fp-contraction=fast from an LLVM TargetOption to a
FastMathFlag in order to fix PR25721.

Differential Revision: https://reviews.llvm.org/D31165

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298961 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed