R600/SI: Use mad for fsub + fmul

We can use a negate source modifier to match
this for fsub.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216735 91177308-0d34-0410-b5e6-96231b3b80d8
Make fabs safe to speculatively execute

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216736 91177308-0d34-0410-b5e6-96231b3b80d8