R600/SI: Add FMA pattern

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188135 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: FMA is faster than fmul and fadd for f64

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188136 91177308-0d34-0410-b5e6-96231b3b80d8