JIT: Don't run optSetBlockWeights when we have PGO data (#111764)

Part of #107749. For PGO scenarios, we plan to rely entirely on profile synthesis to propagate profile-derived weights throughout the flowgraph. This is the first step in transitioning away from the existing weight propagation heuristics. We want to keep these around for non-PGO scenarios for now, though ideally, we'll eventually be able to replace these entirely with profile synthesis (plus branch prediction heuristics when we don't have profile data), and consolidate how the profile is maintained for PGO and non-PGO scenarios.

src/coreclr/jit/optimizer.cpp

@@ -40,9 +40,20 @@ DataFlow::DataFlow(Compiler* pCompiler)
 PhaseStatus Compiler::optSetBlockWeights()
 {
     noway_assert(opts.OptimizationEnabled());
+    assert(m_dfsTree != nullptr);
+    const bool usingProfileWeights = fgIsUsingProfileWeights();
+
+    // Rely on profile synthesis to propagate weights when we have PGO data.
+    // TODO: Replace optSetBlockWeights with profile synthesis entirely.
+    if (usingProfileWeights)
+    {
+        // Leave breadcrumb for loop alignment
+        fgHasLoops = m_dfsTree->HasCycle();
+        return PhaseStatus::MODIFIED_NOTHING;
+    }
+
     bool madeChanges = false;
 
-    assert(m_dfsTree != nullptr);
     if (m_domTree == nullptr)
     {
         m_domTree = FlowGraphDominatorTree::Build(m_dfsTree);
@@ -59,8 +70,7 @@ PhaseStatus Compiler::optSetBlockWeights()
         optFindAndScaleGeneralLoopBlocks();
     }
 
-    bool       firstBBDominatesAllReturns = true;
-    const bool usingProfileWeights        = fgIsUsingProfileWeights();
+    bool firstBBDominatesAllReturns = true;
 
     fgComputeReturnBlocks();