[llvm] [NFC][SROA] Clean up rewritePartition type selection process (PR #169106)

[Alex MacLean via llvm-commits]

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

LGTM. Will land tomorrow per @YonahGoldberg's offline request, unless anyone else has concerns with this change. 

https://github.com/llvm/llvm-project/pull/169106