[DAGCombiner] Initial support for the fast-math flag contract

[DAGCombiner] Initial support for the fast-math flag contract

Now alternatively to the TargetOption.AllowFPOpFusion global flag, FMUL->FADD
can also use the per operation FMF to allow fusion.

The idea here is not to port everything to the new scheme (e.g. fused
multiply-and-sub will be ported later) but that this work all the way from
clang.

The transformation is conditionalized on *both* the FADD and the FMUL having
the FMF contract flag.

Differential Revision: https://reviews.llvm.org/D31169

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299096 91177308-0d34-0410-b5e6-96231b3b80d8