JIT: Use fgCalledCount for OSR method entry weight (#112662)

Follow-up to #112499. By ensuring OSR method entry blocks' weights match fgCalledCount during entry canonicalization, we can guarantee that later runs of profile synthesis won't modify fgCalledCount. This should unblock #111915 (comment).

src/coreclr/jit/fgbasic.cpp

@@ -4046,18 +4046,13 @@ void Compiler::fgFixEntryFlowForOSR()
 
     fgRedirectTargetEdge(fgFirstBB, fgOSREntryBB);
 
-    // We don't know the right weight for this block, since
-    // execution of the method was interrupted within the
-    // loop containing fgOSREntryBB.
-    //
-    // A plausible guess might be to sum the non-backedge
-    // weights of fgOSREntryBB and use those, but we don't
-    // have edge weights available yet. Note that might be
-    // an underestimate.
-    //
-    // For now we just guess that the loop will execute 100x.
-    //
-    fgFirstBB->inheritWeightPercentage(fgOSREntryBB, 1);
+    fgFirstBB->bbWeight = fgCalledCount;
+    fgFirstBB->CopyFlags(fgEntryBB, BBF_PROF_WEIGHT);
+
+    if (fgCalledCount == BB_ZERO_WEIGHT)
+    {
+        fgFirstBB->bbSetRunRarely();
+    }
 
     JITDUMP("OSR: redirecting flow at method entry from " FMT_BB " to OSR entry " FMT_BB " for the importer\n",
             fgFirstBB->bbNum, fgOSREntryBB->bbNum);