[AArch64][GlobalISel] Optimize compare and branch cases with G_INTTOPTR and unknown values.

Since we have distinct types for pointers and scalars, G_INTTOPTRs can sometimes
obstruct attempts to find constant source values. These usually come about when
try to do some kind of null pointer check. Teaching getConstantVRegValWithLookThrough
about this operation allows the CBZ/CBNZ optimization to catch more cases.

This change also improves the case where we can't find a constant source at all.
Previously we would emit a cmp, cset and tbnz for that. Now we try to just emit
a cmp and conditional branch, saving an instruction.

The cumulative code size improvement of this change plus D64354 is 5.5% geomean
on arm64 CTMark -O0.

Differential Revision: https://reviews.llvm.org/D64377

llvm-svn: 365690