[AArch64] Improve the readability of the ld/st optimization pass. NFC.

In this context, MI is an add/sub instruction not a loads/store.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248540 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed