[SROA] rewritePartition()/findCommonType(): if uses have conflicting type, try...

[SROA] rewritePartition()/findCommonType(): if uses have conflicting type, try getTypePartition() before falling back to largest integral use type (PR47592)

And another step towards transformss not introducing inttoptr and/or
ptrtoint casts that weren't there already.

In this case, when load/store uses have conflicting types,
instead of falling back to the iN, we can try to use allocated sub-type.
As disscussed, this isn't the best idea overall (we shouldn't rely on
allocated type), but it works fine as a temporary measure.

I've measured, and @ `-O3` as of vanilla llvm test-suite + RawSpeed,
this results in +0.05% more bitcasts, -5.51% less inttoptr
and -1.05% less ptrtoint (at the end of middle-end opt pipeline)

See https://bugs.llvm.org/show_bug.cgi?id=47592

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D88788