[AArch64] Swap comparison operands if that enables some folding.

Summary:
AArch64 can fold some shift+extend operations on the RHS operand of
comparisons, so swap the operands if that makes sense.

This provides a fix for https://bugs.llvm.org/show_bug.cgi?id=38751

Reviewers: efriedma, t.p.northover, javed.absar

Subscribers: mcrosier, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D53067

llvm-svn: 344439