[AArch64] MachineCombiner FMA matching. NFC.
[AArch64] MachineCombiner FMA matching. NFC.

Follow-up of rL371321 that added some more FP16 FMA patterns, and an attempt to
reduce the copy-pasting and make this more readable.

Differential Revision: https://reviews.llvm.org/D67403

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371818 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm