Simplify x+0 to x in unsafe-fp-math mode. This avoids a bunch of
redundant work in many cases, because in unsafe-fp-math mode,
ISD::FADD with a constant is considered free to negate, so the
DAGCombiner often negates x+0 to -0-x thinking it's free, when
in reality the end result is -x, which is more expensive than x.
Also, combine x*0 to 0.
This fixes PR3374.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62789 91177308-0d34-0410-b5e6-96231b3b80d8
