[GlobalISel][AArch64][NFC] Use getDefIgnoringCopies from Utils where we can

There are a few places where we walk over copies throughout
AArch64InstructionSelector.cpp. In Utils, there's a function that does exactly
this which we can use instead.

Note that the utility function works with the case where we run into a COPY
from a physical register. We've run into bugs with this a couple times, so using
it should defend us from similar future bugs.

Also update opt-fold-compare.mir to show that we still handle physical registers
properly.

Differential Revision: https://reviews.llvm.org/D64513

llvm-svn: 365683

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