[mlir:PassTiming] Always use parentInfo for determining pipeline parent scope

[rriddle]

Summary

This fixes a bug where, depending on thread usage, a pipeline may be
misattributed to a timer that wasn't it's parent.

mlir/lib/Pass/PassTiming.cpp

struct PassTiming : public PassInstrumentation {
   // Pipeline
   //===--------------------------------------------------------------------===//
 
   void runBeforePipeline(Optional<OperationName> name,
                          const PipelineParentInfo &parentInfo) override {
     auto tid = llvm::get_threadid();
     auto &activeTimers = activeThreadTimers[tid];
 
+    // Find the parent scope, either using the parent info or the root scope
+    // (e.g. in the case of the top-level pipeline).
     TimingScope *parentScope;
-    if (activeTimers.empty()) {
-      auto it = parentTimerIndices.find(parentInfo);
-      if (it != parentTimerIndices.end())
-        parentScope =
-            &activeThreadTimers[parentInfo.parentThreadID][it->second];
-      else
-        parentScope = &rootScope;
-    } else {
-      parentScope = &activeTimers.back();
-    }
+      parentScope = &activeThreadTimers[parentInfo.parentThreadID][it->second];
 
     // Use nullptr to anchor op-agnostic pipelines, otherwise use the name of
     // the operation.
     const void *timerId = name ? name->getAsOpaquePointer() : nullptr;
     activeTimers.push_back(parentScope->nest(timerId, [name] {
       return ("'" + (name ? name->getStringRef() : "any") + "' Pipeline").str();
     }));
   }