[EH][GC] Fix nested pop after removing ref.cast

[aheejin]

ref.cast can be statically removed when the ref's type is a subtype of
the intended RTT type and either of --ignore-implicit-traps or
--traps-never-happen is given:

binaryen/src/passes/OptimizeInstructions.cpp

Lines 1603 to 1624 in 083ab98

	if (passOptions.ignoreImplicitTraps || passOptions.trapsNeverHappen ||
	!curr->rtt) {
	// Aside from the issue of type incompatibility as mentioned above, the
	// cast can trap if the types *are* compatible but it happens to be the
	// case at runtime that the value is not of the desired subtype. If we
	// do not consider such traps possible, we can ignore that. (Note,
	// though, that we cannot do this if we cannot replace the current type
	// with the reference's type.) We can also do this if this is a static
	// cast: in that case, all we need to know about are the types.
	if (HeapType::isSubType(curr->ref->type.getHeapType(), intendedType)) {
	if (curr->rtt) {
	replaceCurrent(getResultOfFirst(curr->ref,
	builder.makeDrop(curr->rtt),
	getFunction(),
	getModule(),
	passOptions));
	} else {
	replaceCurrent(curr->ref);
	}
	return;
	}
	}

Some more context: #4097 (comment)

But this can create a block in which a pop is nested, which makes the
catch invalid. The test in this PR is the same as the example given by
@kripken in #4237. This calls the fixup function
EHUtils::handleBlockNestedPops at the end of the pass to fix this.

Also, because this pass creates a lot of blocks in other patterns, I
think it is possible there can be other patterns to cause this kind of
pop nesting.