[GlobalISel][AArch64] Use getConstantVRegValWithLookThrough for selectArithImmed

Instead of just stopping to see if we have a G_CONSTANT, instead, look through
G_TRUNCs, G_SEXTs, and G_ZEXTs.

This gives an average ~1.3% code size improvement on CINT2000 at -O3.

Differential Revision: https://reviews.llvm.org/D64108

llvm-svn: 365063