[llvm] [CodeGen] Add a helper class to reuse `expandMBB`. NFC. (PR #70325)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

I agree - I'm not sure if this patch is that useful, it avoids a little duplicate code but we have plenty of cases where targets duplicate code (either by design or due to one target copying another implementation).

https://github.com/llvm/llvm-project/pull/70325