Mark arrays containing partial arrays as partial. This was already
done for the ADD_ARRAY_ELEMENT case, but not for ASSIGN_DIM.

SCCP did not handle array ASSIGN_* with a BOT operand correctly.

Correctly handle the case of duplicate predecessors, by removing the
duplicate predecessor and corresponding phi node operands.

For the future, it would be better to instead allow duplicate
predecessors and avoid this kind of fragile code...

Instead of juggling with this problem during literal compaction,
make sure that we always initialize Z_EXTRA for literals, which
seems like the more robust solution.

Remove invalid assertion: A block can have multiple switch frees,
so if we don't do live range block splitting, it is not necessarily
true that the free is located at the start of a block.

Weird that this worked for so long, probably because nearly all
ext/standard functions use fast ZPP rather than ordinary ZPP.