[AArch64][GlobalISel] Optimize conjunctions of compares to conditional compares.

This is a partial port of the same optimization from AArch64ISelLowering,
although the original handles more cases when generating regular compares
instead of this one which just does it when selecting G_SELECTs.

For more detailed comments see the original comments for
emitConditionalComparison() in AArch64ISelLowering.

Gives minor code size improvements.

Differential Revision: https://reviews.llvm.org/D117166