[SROA] `isVectorPromotionViable()`: memory intrinsics operate on vectors of bytes

Now, there's a big caveat here - these bytes
are abstract bytes, not the i8 we have in LLVM,
so strictly speaking this is not exactly legal,
see e.g. https://github.com/AliveToolkit/alive2/issues/860
^ the "bytes" "could" have been a pointer,
and loading it as an integer inserts an implicit ptrtoint.

But at the same time,
InstCombine's `InstCombinerImpl::SimplifyAnyMemTransfer()`
would expand a memtransfer of 1/2/4/8 bytes
into integer-typed load+store,
so this isn't exactly a new problem.

Note that in memory, poison is byte-wise,
so we really can't widen elements,
but SROA seems to be inconsistent here.

Fixes #59116.