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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

An llc test that falls-back to SDAG would be fine as well, so long as it doesn't crash. The mir test has advantages that it can be specific to this issue though, they are good at testing the edge cases.

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