[PowerPC] Adjust the naming and operand order of fnmsub patterns

Summary:
This is follow up patch of https://reviews.llvm.org/D67595.
Adjust naming and the Commutable operands for additional patterns
to make it easier to read.

The testcase update also show that we can save some unecessary fmr as
well.

Reviewers: #powerpc, steven.zhang, hfinkel, nemanjai

Reviewed By: #powerpc, nemanjai

Subscribers: wuzish, hiraditya, kbarton, MaskRay, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68112

llvm-svn: 373652