[AArch64][GlobalISel] Implement selection og G_MERGE of two s32s into s64.

This code pattern is an unfortunate side effect of the way some types get split
at call lowering. Ideally we'd either not generate it at all or combine it away
in the legalizer artifact combiner.

Until then, add selection support anyway which is a significant proportion of
our current fallbacks on CTMark.

rdar://46491420

llvm-svn: 349712