[llvm] [AArch64][GlobalISel] Check unmergeSrc is a vector in matchCombineBuildUnmerge (PR #168692)

[Nathan Corbyn via llvm-commits]

https://github.com/cofibrant approved this pull request.

Assuming CI passes, LGTM. Let's see what @davemgreen says before merging, though.

https://github.com/llvm/llvm-project/pull/168692