[X86][FMA] Optimize FNEG(FMA) Patterns

X86 needs to use its own FMA opcodes, preventing the standard FNEG(FMA) pattern table recognition method used by other platforms. This patch adds support for lowering FNEG(FMA(X,Y,Z)) into a single suitably negated FMA instruction.

Fix for PR24364

Differential Revision: http://reviews.llvm.org/D14906

llvm-svn: 254016