R600/SI: Fix using mad with multiplies by 2

These turn into fadds, so combine them into the target
mad node.

fadd (fadd (a, a), b) -> mad 2.0, a, b

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218608 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Also fix fsub + fadd a, a to mad combines

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218609 91177308-0d34-0410-b5e6-96231b3b80d8