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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236546 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -2660,8 +2660,9 @@ SDValue TargetLowering::BuildExactSDIV(SDValue Op1, SDValue Op2, SDLoc dl,
     // TODO: For UDIV use SRL instead of SRA.
     SDValue Amt = DAG.getConstant(ShAmt, dl,
                                   getShiftAmountTy(Op1.getValueType()));
-    Op1 = DAG.getNode(ISD::SRA, dl, Op1.getValueType(), Op1, Amt, false, false,
-                      true);
+    SDNodeFlags Flags;
+    Flags.setExact(true);
+    Op1 = DAG.getNode(ISD::SRA, dl, Op1.getValueType(), Op1, Amt, &Flags);
     d = d.ashr(ShAmt);
   }