Fix issue #1794388.

Fix issue #1794388.

In particular, when encountering a known-null being used as an array,
always emit an instruction that's compatible with the element type
expected/encountered in context. This will sometimes lead to
surprising instruction choices, such as "aget-byte" when the original
source used a boolean[], but in all of these cases the output is valid
and behaves correctly, where "correctly" in this case means (a)
passing verification, and (b) throwing a NullPointerException if ever
executed.

The test case (dx test #111) reflects unsurprising expectations and
so needs updating, which I will do in a follow-up to this patch.

I know this makes me an awful git user, but I fixed some comments and
whitespace while I was in the territory and *didn't* turn these into
separate commits.