[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, 10 insertions, 10 deletions

diff --git a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
index f8e7778c5..756b692c7 100644
--- a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
+++ b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
@@ -3,20 +3,20 @@
  (type $i32_=>_none (func (param i32)))
  (event $e0 (attr 0) (param i32))
  (func $eh
-  try $try
+  try $l0
    i32.const 0
    throw $e0
   catch $e0
    
    drop
   catch_all
-   rethrow 0
+   rethrow $l0
   end
-  try $l0
-   try $try0
+  try $l00
+   try $try
     i32.const 0
     throw $e0
-   delegate $l0
+   delegate $l00
    unreachable
   catch_all
    nop
@@ -31,7 +31,7 @@
  (type $i32_=>_none (func (param i32)))
  (event $e0 (attr 0) (param i32))
  (func $eh (; has Stack IR ;)
-  (try $try
+  (try $l0
    (do
     (throw $e0
      (i32.const 0)
@@ -43,18 +43,18 @@
     )
    )
    (catch_all
-    (rethrow 0)
+    (rethrow $l0)
    )
   )
-  (try $l0
+  (try $l00
    (do
-    (try $try0
+    (try $try
      (do
       (throw $e0
        (i32.const 0)
       )
      )
-     (delegate $l0)
+     (delegate $l00)
     )
    )
    (catch_all