[Wasm GC] Heap2Local: Replace the allocation with null (#3893)

Previously we would try to stop using the allocation as much as possible,
for example not writing it to locals any more, and leaving it to other passes
to actually remove it (and remove gets of those locals etc.). This seemed
simpler and more modular, but does not actually work in some cases as
the fuzzer has found. Specifically, if we stop writing our allocation to
locals, then if we do a (ref.as_non_null (local.get ..)) of that, then we
will trap on the null present in the local.

Instead, this changes our rewriting to do slightly more work, but it is
simpler in the end. We replace the allocation with a null, and replace
all the places that use it accordingly, for example, updating types to be
nullable, and removing RefAsNonNulls, etc. This literally gets rid of the
allocation and all the places it flows to (leaving less for other passes to
do later).

0 files changed, 0 insertions, 0 deletions