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

[Ryan Cowan via llvm-commits]

HolyMolyCowMan wrote:

I could add the MIR & only run the aarch64-postlegalizer-combiner pass to show the crash no longer happens, but there are other issues with selection for that reproducer.

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