Code pushing support for br_on_exn (#2660)

Like `br_if`, `br_on_exn` is a conditional branch and across which code
can be pushed past when conditions are satisfied.

Also adds a few lines of comments and NFC changes in a couple places.
Changes in Vacuum are NFC because they were being handled in `default:`
in the same way anyway, but I added them to be more explicit and
consistent with existing code.

2 files changed, 31 insertions, 0 deletions

diff --git a/test/passes/code-pushing_all-features.txt b/test/passes/code-pushing_all-features.txt
index 230a9006d..6cb241e26 100644
--- a/test/passes/code-pushing_all-features.txt
+++ b/test/passes/code-pushing_all-features.txt
@@ -94,4 +94,21 @@
    )
   )
  )
+ (func $push-past-br-on-exn (; 4 ;)
+  (local $x i32)
+  (local $y exnref)
+  (drop
+   (block $out (result i32)
+    (drop
+     (br_on_exn $out $e
+      (local.get $y)
+     )
+    )
+    (local.set $x
+     (i32.const 1)
+    )
+    (local.get $x)
+   )
+  )
+ )
 )
diff --git a/test/passes/code-pushing_all-features.wast b/test/passes/code-pushing_all-features.wast
index f24456c6d..03debf139 100644
--- a/test/passes/code-pushing_all-features.wast
+++ b/test/passes/code-pushing_all-features.wast
@@ -60,4 +60,18 @@
       (drop (local.get $x))
     )
   )
+
+  (func $push-past-br-on-exn
+    (local $x i32)
+    (local $y exnref)
+    (drop
+      (block $out (result i32)
+        (local.set $x (i32.const 1))
+        (drop
+          (br_on_exn $out $e (local.get $y))
+        )
+        (local.get $x)
+      )
+    )
+  )
 )