[GlobalISel][AArch64] Fix contract cross-bank copies with SIMD instructions

contractCrossBankCopyIntoStore() finds the instruction defines the
source register and uses its output to replace the register. There are,
however, instructions that have multiple outputs, e.g. G_UNMERGE_VALUES.
Current implementation hardcodes to operand 0 and has no way of knowing
which output should be used.

This change adds another function to directly return the register that
is the source of the register and use that for folding.

This fixes https://bugs.llvm.org/show_bug.cgi?id=44783

Differential Revision: https://reviews.llvm.org/D74005