RemoveUnusedBrs: Avoid an error on loops with unreachable ifs (#7156)

We normally like to move brs after ifs into the if, when in a loop:

(loop $loop
  (if
    ..
    (unreachable)
    (code)
  )
  (br $loop)
)

=>

(loop $loop
  (if
    ..
    (unreachable)
    (block
      (code)
      (br $loop) ;; moved in
    )
  )
)

However this may be invalid to do if the if condition is unreachable, as
then one arm may be concrete (`code` in the example could be an `i32`,
for example). As this is dead code anyhow, leave it for DCE.

1 files changed, 7 insertions, 3 deletions

diff --git a/src/passes/RemoveUnusedBrs.cpp b/src/passes/RemoveUnusedBrs.cpp
index 32f7bd48a..47a1f1c65 100644
--- a/src/passes/RemoveUnusedBrs.cpp
+++ b/src/passes/RemoveUnusedBrs.cpp
@@ -621,9 +621,13 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
             block->finalize();
             return true;
           }
-        } else {
-          // this is already an if-else. if one side is a dead end, we can
-          // append to the other, if there is no returned value to concern us
+        } else if (iff->condition->type != Type::unreachable) {
+          // This is already an if-else. If one side is a dead end, we can
+          // append to the other, if there is no returned value to concern us.
+          // Note that we skip ifs with unreachable conditions, as they are dead
+          // code that DCE can remove, and modifying them can lead to errors
+          // (one of the arms may still be concrete, in which case appending to
+          // it would be invalid).
 
           // can't be, since in the middle of a block
           assert(!iff->type.isConcrete());