Ensure symmetric results in PossibleConstantValues (#4662)

Previously we could return different results depending on the order we
noted things:

note(anyref.null);
note(funcref.null);
get() => anyref.null

note(funcref.null);
note(anyref.null);
get() => funcref.null

This is correct, as nulls are equal anyhow, and any could be used in
the location we are optimizing. However, it can lead to nondeterminism
if the caller's order of notes is nondeterministic. That is the case in
DeadArgumentElimination, where we scan functions in parallel, then
merge them without special ordering.

To fix this, make the note operation symmetric. That seems simplest and
least likely to be confusing. We can use the LUB to do that.

To avoid duplicating the null logic, refactor note() to use combine().

0 files changed, 0 insertions, 0 deletions