[DAGCombiner] exclude x*2.0 from normal negation profitability rules

This is the codegen part of fixing:
https://bugs.llvm.org/show_bug.cgi?id=32939

Even with the optimal/canonical IR that is ideally created by D65954,
we would reverse that transform in DAGCombiner and end up with the same
asm on AArch64 or x86.

I see 2 options for trying to correct this:

  1. Limit isNegatibleForFree() by special-casing the fmul pattern (this patch).
  2. Avoid creating (fmul X, 2.0) in the 1st place by adding a special-case
     transform to SelectionDAG::getNode() and/or SelectionDAGBuilder::visitFMul()
     that matches the transform done by DAGCombiner.

This seems like the less intrusive patch, but if there's some other reason to
prefer 1 option over the other, we can change to the other option.

Differential Revision: https://reviews.llvm.org/D66016

llvm-svn: 368490