[InterleavedAccessPass] Don't increase the number of bytes loaded.

Even if the interleaving transform would otherwise be legal, we shouldn't
introduce an interleaved load that is wider than the original load: it might
have undefined behavior.

It might be possible to perform some sort of mask-narrowing transform in
some cases (using a narrower interleaved load, then extending the
results using shufflevectors).  But I haven't tried to implement that,
at least for now.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41245 .

Differential Revision: https://reviews.llvm.org/D59954

llvm-svn: 357212