[llvm][CodeGen] Check for memory instructions when querying for alias status

Summary:
Add a check to make sure that MachineInstr::mayAlias returns prematurely if at least one of its instruction parameters does not access memory. This prevents calls to TargetInstrInfo::areMemAccessesTriviallyDisjoint with incompatible instructions.

A side effect of this change is to render the mayAlias helper in the AArch64 load/store optimizer obsolete. We can now directly call the MachineInstr::mayAlias member function.

Reviewers: hfinkel, t.p.northover, mcrosier, eli.friedman, efriedma

Reviewed By: efriedma

Subscribers: efriedma, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78823