Fix flow reset during throw => break opts in RemoveUnusedBrs (#6993)

#6980 was missing the logic to reset flows after replacing a throw. The process
of replacing the throw introduces new code and in particular a drop, which
blocks branches from flowing to their targets.

In the testcase here, the br was turned into nop before this fix.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/passes/RemoveUnusedBrs.cpp b/src/passes/RemoveUnusedBrs.cpp
index c2370aca6..c2f470d80 100644
--- a/src/passes/RemoveUnusedBrs.cpp
+++ b/src/passes/RemoveUnusedBrs.cpp
@@ -518,6 +518,10 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
             auto* rep = getDroppedChildrenAndAppend(
               curr, wasm, getPassOptions(), br, DropMode::IgnoreParentEffects);
             replaceCurrent(rep);
+            // We modified the code here and may have added a drop, etc., so
+            // stop the flow (rather than re-scan it somehow). We leave
+            // optimizing anything that flows out for later iterations.
+            stopFlow();
           }
 
           // Return even if we did not optimize: we found our tag was caught.