[BPF] Don't fetch alignment of llvm.preserve.union.access.index argument

The way the alignment is determined here is not compatible with
opaque pointers -- we'd have to thread it through using either
align or elementtype attributes.

However, as far as I can tell this alignment is actually never
used for this particular intrinsic, so I've dropped the assignment
entirely and converted RecordAlignment to MaybeAlign, so we get an
assertion failure if it does end up being used.

Differential Revision: https://reviews.llvm.org/D122379