[llvm] [AArch64][GlobalISel] Lower G_BUILD_VECTOR to G_INSERT_VECTOR_ELT (PR #105686)

[David Green via llvm-commits]

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

This LGTM, and it seems to fit into the existing design of doing certain lowering post-legalization. We went through the test changes and decided they were improvements or should be optimized in some other way (such as #105692).

https://github.com/llvm/llvm-project/pull/105686