Fix dotnet-pgo SPGO crash in FlowSmoothing.MakeGraphFeasible

FlowSmoothing.MakeGraphFeasible throws InvalidOperationException
("Stack empty") when a method's flow graph has a node with positive
sample count that doesn't reach an exit node. This crashes the
entire SPGO pass in SmoothAllProfiles, losing all attributed samples.

Wrap SmoothFlow in a try-catch so one problematic method doesn't
prevent SPGO data from being generated for all other methods.

Observed when processing perfcollect .trace.zip with ~143K
successfully attributed samples — all lost due to one bad method.

Author: benaadams

src/coreclr/tools/dotnet-pgo/SPGO/SampleCorrelator.cs

@@ -66,7 +66,7 @@ public SampleProfile GetProfile(MethodDesc md)
         public void SmoothAllProfiles()
         {
             foreach (PerMethodInfo pmi in _methodInf.Values)
-                pmi.Profile.SmoothFlow();
+                try { pmi.Profile.SmoothFlow(); } catch (Exception) { /* Skip methods with disconnected flow graphs */ }
         }
 
         public void AttributeSamplesToIP(ulong ip, long numSamples)