DebugInfo: Don't trample in replaceCurrent() (#5915)

Copy the old expression's debug info if the new has none. But if the new has
its own, trust that.

Followup to #5914

Author: kripken

src/wasm-traversal.h

@@ -124,7 +124,12 @@ struct Walker : public VisitorType {
     // Copy debug info, if present.
     if (currFunction) {
       auto& debugLocations = currFunction->debugLocations;
-      if (!debugLocations.empty()) {
+      // Early exit if there is no debug info at all. Also, leave if we already
+      // have debug info on the new expression, which we don't want to trample:
+      // if there is no debug info we do want to copy, as a replacement
+      // operation suggests the new code plays the same role (it is an optimized
+      // version of the old), but if the code is already annotated, trust that.
+      if (!debugLocations.empty() && !debugLocations.count(expression)) {
         auto* curr = getCurrent();
         auto iter = debugLocations.find(curr);
         if (iter != debugLocations.end()) {

test/lit/debug/replace-keep.wat

@@ -37,4 +37,36 @@
       )
     )
   )
+
+  ;; CHECK:      (func $test-no-trample
+  ;; CHECK-NEXT:  (local $temp i32)
+  ;; CHECK-NEXT:  [none] ;;@ src.cpp:300:3
+  ;; CHECK-NEXT:  [none](block
+  ;; CHECK-NEXT:   [none] ;;@ src.cpp:400:4
+  ;; CHECK-NEXT:   (call $test)
+  ;; CHECK-NEXT:   [none] ;;@ src.cpp:200:2
+  ;; CHECK-NEXT:   (local.set $temp
+  ;; CHECK-NEXT:    [i32] ;;@ src.cpp:500:5
+  ;; CHECK-NEXT:    (i32.const 1)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  ) ;; end block
+  ;; CHECK-NEXT:  ;;@ src.cpp:200:2
+  ;; CHECK-NEXT: )
+  (func $test-no-trample
+    (local $temp i32)
+
+    ;; As above, but now the inner block has debug info (300), which should not
+    ;; be trampled as it is moved outside.
+
+    ;;@ src.cpp:200:2
+    (local.set $temp
+      ;;@ src.cpp:300:3
+      (block (result i32)
+        ;;@ src.cpp:400:4
+        (call $test)
+        ;;@ src.cpp:500:5
+        (i32.const 1)
+      )
+    )
+  )
 )