The issue was the renderer for x86 addressing modes assumes native size
registers, but we were passing in a possibly-smaller index in
conjunction with a native-sized base pointer.

The easist thing to do is just extend the register first.

I also changed the other NGC backends implementing jump tables
accordingly. On one hand, I think PowerPC and Sparc don't have the small
sub-registers anyways so there is less to worry about. On the other
hand, to the extent that's true the zero extension can become a no-op.

I should give credit where it's due: @hsyl20 really did all the work for
me in
ghc/ghc!4717 (comment 355874),
but I was daft and missed the "Oops" and so ended up spending a silly
amount of time putting it all back together myself.

The unregisterised backend change is a bit different, because here we
are translating the actual case not a jump table, and the fix is to
handle right-sized literals not addressing modes. But it makes sense to
include here too because it's the same change in the subsequent commit
that exposes both bugs.