Use AllocaInst::getAllocationSize instead of manual size calculations (#176486)

Replace patterns that manually compute allocation sizes by multiplying
getTypeAllocSize(getAllocatedType()) by the array size with calls to the
getAllocationSize(DL) API, which handles this correctly and concisely,
returning nullopt for VLAs.

This fixes several places that were not accounting for array allocations
when computing sizes, simplifies code that was doing this manually, and
adds some explicit isFixed checks where implied convert was being used.

This PR is because now that we have opaque pointers, I hate that some
AllocaInst still has type information being consumed by some passes
instead of just using the size, since passes rarely handle that type
information well or correctly. I hope this will grow into a sequence of
commits to slowly eliminate uses of getAllocatedType from AllocaInst.
And similarly later to remove type information from GlobalValue too (it
can be replaced with just dereferenceable bytes, similar to arguments).

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>