[AArch64] Refactor macro fusion (NFC)

Move checks for each fusion case into separate functions for better
legibility and maintainability.

Differential revision: https://reviews.llvm.org/D43649

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325844 91177308-0d34-0410-b5e6-96231b3b80d8