don't turn untaken br_ifs into ifs in remove-unused-brs

Author: kripken

src/passes/RemoveUnusedBrs.cpp

@@ -481,7 +481,9 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
           // format. We need to flip the condition, which at worst adds 1.
           if (curr->name.is()) {
             auto* br = list[0]->dynCast<Break>();
-            if (br && br->condition && br->name == curr->name) {
+            // we seek a regular br_if; if the type is unreachable that means it is not
+            // actually taken, so ignore
+            if (br && br->condition && br->name == curr->name && br->type != unreachable) {
               assert(!br->value); // can't, it would be dropped or last in the block
               if (BranchUtils::BranchSeeker::countNamed(curr, curr->name) == 1) {
                 // no other breaks to that name, so we can do this

test/passes/remove-unused-brs.txt

@@ -1048,4 +1048,15 @@
    )
   )
  )
+ (func $untaken-br_if-then-if (type $1)
+  (block $label$0
+   (br_if $label$0
+    (unreachable)
+   )
+   (if
+    (i32.const 0)
+    (nop)
+   )
+  )
+ )
 )

test/passes/remove-unused-brs.wast

@@ -932,5 +932,16 @@
     )
    )
   )
+  (func $untaken-br_if-then-if
+   (block $label$0
+    (br_if $label$0
+     (unreachable)
+    )
+    (if
+     (i32.const 0)
+     (nop)
+    )
+   )
+  )
 )