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

[Ryan Cowan via llvm-commits]

HolyMolyCowMan wrote:

I've added the MIR for the reproducer to a test, as well as some basic tests of functionality for completeness' sake.

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