Heap2Local: Handle unreachable ref.test replacement

[kripken]

As with local.get in other cases, here we replace with something
concrete (a const), and we can't do that if we became unreachable.

[tlively]

Can you be more specific about where the validation failure would be? It looks like the constant would flow into the drop, but that would be perfectly valid.

[kripken]

It's the usual issue with local.get, like we do in a few other places,

binaryen/src/passes/Heap2Local.cpp

Lines 978 to 985 in 95d8876

	if (curr->type == Type::unreachable) {
	// We must not modify unreachable code here, as we will replace it with a
	// local.get, which has a concrete type (another option could be to run
	// DCE and not only ReFinalize - DCE will propagate an unreachable out of
	// a concrete block, like we emit here - but we can just ignore such
	// code).
	return;
	}

If we don't skip this code, we emit a block with an unreachable child followed by a concrete one (here, a const). So yes, there is a drop of the block that ends in the constant, but the problem is that the block has the wrong type.