[llvm] [AArch64][GlobalISel] Bitcast and Build Illegal G_CONCAT_VECTOR Instructions (PR #96492)

[Amara Emerson via llvm-commits]

aemerson wrote:

It's been a while and I don't remember the issues from the other patch that motivated these changes.

In general, could you provide some higher level context about what the actual problem is and why other more obvious approaches don't work? That kind of reasoning should also go into the commit or comments.

https://github.com/llvm/llvm-project/pull/96492