[EH] Make rethrow's target a try label (#3568)

I was previously mistaken about `rethrow`'s argument rule and thought
it only counted `catch`'s depth. But it turns out it follows the same
rule `delegate`'s label: the immediate argument follows the same rule as
when computing branch labels, but it only can target `try` labels
(semantically it targets that `try`'s corresponding `catch`); otherwise
it will be a validation failure. Unlike `delegate`, `rethrow`'s label
denotes not where to rethrow, but which exception to rethrow. For
example,
```wasm
try $l0
catch ($l0)
  try $l1
  catch ($l1)
   rethrow $l0 ;; rethrow the exception caught by 'catch ($l0)'
  end
end
```

Refer to this comment for the more detailed informal semantics:
https://github.com/WebAssembly/exception-handling/issues/146#issuecomment-777714491

---

This also reverts some of `delegateTarget` -> `exceptionTarget` changes
done in #3562 in the validator. Label validation rules apply differently
for `delegate` and `rethrow` for try-catch. For example, this is valid:
```wasm
try $l0
  try
  delegate $l0
catch ($l0)
end
```
But this is NOT valid:
```wasm
try $l0
catch ($l0)
  try
  delegate $l0
end
```
So `try`'s label should be used within try-catch range (not catch-end
range) for `delegate`s.

But for the `rethrow` the rule is different. For example, this is valid:
```wasm
try $l0
catch ($l0)
  rethrow $l0
end
```
But this is NOT valid:
```wasm
try $l0
  rethrow $l0
catch ($l0)
end
```
So the `try`'s label should be used within catch-end range instead.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/ir/branch-utils.h b/src/ir/branch-utils.h
index 826483b09..f6c55c563 100644
--- a/src/ir/branch-utils.h
+++ b/src/ir/branch-utils.h
@@ -83,7 +83,7 @@ void operateOnScopeNameUsesAndSentTypes(Expression* expr, T func) {
     } else if (auto* br = expr->dynCast<BrOn>()) {
       func(name, br->getCastType());
     } else {
-      assert(expr->is<Try>()); // delegate
+      assert(expr->is<Try>() || expr->is<Rethrow>()); // delegate or rethrow
     }
   });
 }
@@ -135,14 +135,14 @@ inline bool replacePossibleTarget(Expression* branch, Name from, Name to) {
   return worked;
 }
 
-// Replace all delegate targets within the given AST.
+// Replace all delegate/rethrow targets within the given AST.
 inline void replaceExceptionTargets(Expression* ast, Name from, Name to) {
   struct Replacer
     : public PostWalker<Replacer, UnifiedExpressionVisitor<Replacer>> {
     Name from, to;
     Replacer(Name from, Name to) : from(from), to(to) {}
     void visitExpression(Expression* curr) {
-      if (curr->is<Try>()) {
+      if (curr->is<Try>() || curr->is<Rethrow>()) {
         operateOnScopeNameUses(curr, [&](Name& name) {
           if (name == from) {
             name = to;