[X86] Add support for combining FMSUB/FNMADD/FNMSUB ISD nodes with an fneg input.

Previously we could only negate the FMADD opcodes. This used to be mostly ok when we lowered FMA intrinsics during lowering. But with the move to llvm.fma from target specific intrinsics, we can combine (fneg (fma)) to (fmsub) earlier. So if we start with (fneg (fma (fneg))) we would get stuck at (fmsub (fneg)).

This patch fixes that so we can also combine things like (fmsub (fneg)).

llvm-svn: 336304