Fix interference caused by fmul 2, x -> fadd x, x

If an fmul was introduced by lowering, it wouldn't be folded
into a multiply by a constant since the earlier combine would
have replaced the fmul with the fadd.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216932 91177308-0d34-0410-b5e6-96231b3b80d8