propagate IR-level fast-math-flags to DAG nodes (NFC)

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.

There are 2 structural changes here:

1. The main diff is that we're preparing to extend the optimization
flags to affect more than just binary SDNodes. Eg, IR intrinsics 
( https://llvm.org/bugs/show_bug.cgi?id=21290 ) or non-binop nodes
that don't even exist in IR such as FMA, FNEG, etc.

2. The other change is that we're actually copying the FP fast-math-flags
from the IR instructions to SDNodes. 

Differential Revision: http://reviews.llvm.org/D8900

llvm-svn: 236546