Fix opcode merging in inverted flamegraph view

                                                                                                                                                           Opcodes from multiple call paths were silently dropped, only the
first path's opcodes were kept. Now they're summed correctly when
nodes merge.

Author: ivonastojanovic

Lib/profiling/sampling/_flamegraph_assets/flamegraph.js

@@ -1276,7 +1276,7 @@ function accumulateInvertedNode(parent, stackFrame, leaf, isDifferential) {
       lineno: stackFrame.lineno,
       funcname: stackFrame.funcname,
       source: stackFrame.source,
-      opcodes: stackFrame.opcodes,
+      opcodes: null,
       threads: new Set()
     };
 
@@ -1296,6 +1296,15 @@ function accumulateInvertedNode(parent, stackFrame, leaf, isDifferential) {
   if (leaf.threads) {
     leaf.threads.forEach(t => node.threads.add(t));
   }
+  if (stackFrame.opcodes) {
+    if (!node.opcodes) {
+      node.opcodes = { ...stackFrame.opcodes };
+    } else {
+      for (const [op, count] of Object.entries(stackFrame.opcodes)) {
+        node.opcodes[op] = (node.opcodes[op] || 0) + count;
+      }
+    }
+  }
 
   if (isDifferential) {
     node.baseline += stackFrame.baseline || 0;