[CodeGen] Use RegUnits to track register aliases (NFC)

Summary: Use RegUnits to track register aliases in PostRASink and AArch64LoadStoreOptimizer.

Reviewers: thegameg, mcrosier, gberry, qcolombet, sebpop, MatzeB, t.p.northover, javed.absar

Reviewed By: thegameg, sebpop

Subscribers: javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D45695

llvm-svn: 331066