[X86] Add combination for fma and fneg on X86 under strict FP.

Summary: X86 has instructions to calculate fma and fneg at the same time. But we combine the fneg and fma only when fneg is the source operand under strict FP.

Reviewers: craig.topper, andrew.w.kaylor, uweigand, RKSimon, LiuChen3

Subscribers: LuoYuanke, llvm-commits, cfe-commits, jdoerfert, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72824