fix merge-blocks bug with replacing an unreachable block with a concrete final element (which is never reached)

Author: kripken

src/passes/MergeBlocks.cpp

@@ -294,11 +294,12 @@ struct MergeBlocks : public WalkerPass<PostWalker<MergeBlocks>> {
           // here
           return outer;
         }
+        // we are going to replace the block with the final element, so they should
+        // be identically typed
+        if (block->type != back->type) {
+          return outer;
+        }
         child = back;
-        // we modified child (which is a reference to a pointer), which modifies curr, which might change its type
-        // (e.g. (drop (block (result i32) .. (unreachable)))
-        // the child was a block of i32, and is being replaced with an unreachable, so the
-        // parent will likely need to be unreachable too
         if (outer == nullptr) {
           // reuse the block, move it out
           block->list.back() = curr;

test/passes/remove-unused-names_merge-blocks.txt

@@ -892,4 +892,18 @@
    )
   )
  )
+ (func $drop-unreachable-block-with-concrete-final (type $3)
+  (drop
+   (block
+    (drop
+     (block
+      (drop
+       (return)
+      )
+     )
+    )
+    (i32.const -452)
+   )
+  )
+ )
 )

test/passes/remove-unused-names_merge-blocks.wast

@@ -1054,4 +1054,18 @@
    )
   )
  )
+ (func $drop-unreachable-block-with-concrete-final
+  (drop
+   (block
+    (drop
+     (block
+      (drop
+       (return)
+      )
+     )
+    )
+    (i32.const -452)
+   )
+  )
+ )
 )