JIT: initial work on general count reconstruction (#99992)

Implements a Gauss-Seidel solver for cases where method have irreducible loops.

Author: AndyAyersMS

src/coreclr/jit/fgprofile.cpp

@@ -2949,8 +2949,8 @@ PhaseStatus Compiler::fgIncorporateProfileData()
         //
         if (fgPgoHaveWeights && !dataIsGood)
         {
-            JITDUMP("\nIncorporated count data had inconsistencies; blending profile...\n");
-            ProfileSynthesis::Run(this, ProfileSynthesisOption::BlendLikelihoods);
+            JITDUMP("\nIncorporated count data had inconsistencies; repairing profile...\n");
+            ProfileSynthesis::Run(this, ProfileSynthesisOption::RepairLikelihoods);
         }
     }
 
@@ -3266,15 +3266,9 @@ class EfficientEdgeCountReconstructor : public SpanningTreeVisitor
 
     // Are there are reparable issues with the reconstruction?
     //
-    // Ideally we'd also have || !m_negativeCount here, but this
-    // leads to lots of diffs in async methods.
-    //
-    // Looks like we might first need to resolve reconstruction
-    // shortcomings with irreducible loops.
-    //
     bool IsGood() const
     {
-        return !m_entryWeightZero;
+        return !(m_entryWeightZero || m_negativeCount);
     }
 
     void VisitBlock(BasicBlock*) override