[llvm] [SROA] Fix incorrect offsets for structured binding variables (PR #69007)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

Seems good in general although I have some test questions.

I think the only other concern is that there's some fiddly logic in `splitAlloca` to detect "too big" "too small" allocas, is there definitely test coverage for that that's ensuring we're getting it right?

https://github.com/llvm/llvm-project/pull/69007