propagate IR-level fast-math-flags to DAG nodes; 2nd try; NFC

propagate IR-level fast-math-flags to DAG nodes; 2nd try; NFC

This is a less ambitious version of:
http://reviews.llvm.org/rL236546

because that was reverted in:
http://reviews.llvm.org/rL236600

because it caused memory corruption that wasn't related to FMF
but was actually due to making nodes with 2 operands derive from a
plain SDNode rather than a BinarySDNode.

This patch adds the minimum plumbing necessary to use IR-level
fast-math-flags (FMF) in the backend without actually using
them for anything yet. This is a follow-on to:
http://reviews.llvm.org/rL235997

...which split the existing nsw / nuw / exact flags and FMF
into their own struct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237046 91177308-0d34-0410-b5e6-96231b3b80d8