[llvm] [SROA] Canonicalize homogeneous structs into fixed vectors (PR #165159)

[Yonah Goldberg via llvm-commits]

YonahGoldberg wrote:

ik you didn't ask me to review, but I recently touched the type selection process so I'll take a look this week. In the meantime though, can you (1) rebase (idk why it's not showing merge conflicts, this should have conflicts with my most recent SROA change) and (2) use `llvm/utils/update_test_checks.py` to generate the CHECKs.

https://github.com/llvm/llvm-project/pull/165159