[llvm] [Matrix] Propagate shape information through PHI insts (PR #141681)

[Jon Roelofs via llvm-commits]

jroelofs wrote:

> ⚠️ undef deprecator found issues in your code. ⚠️

I am going to ignore this one. It should be appropriate for a lowering pass to handle undef, and for tests to check that behavior. When `undef` is _actually_ deprecated, these should be easy to remove.

https://github.com/llvm/llvm-project/pull/141681