JIT: update block weight for uncond to cond flow opt (#98324)

This optimization duplicates code and flow in a BBJ_COND successor into one of its
preds; as a result the weight of the successor should decrease.

Fixes some issues seen with odd perf scores in the ML/CSE experiment.

Contributes to #93020

Author: AndyAyersMS

src/coreclr/jit/fgopt.cpp

@@ -2470,27 +2470,32 @@ bool Compiler::fgOptimizeUncondBranchToSimpleCond(BasicBlock* block, BasicBlock*
         fgInsertStmtAtEnd(block, cloneStmt);
     }
 
-    // add an unconditional block after this block to jump to the target block's fallthrough block
+    // Fix up block's flow.
+    // Assume edge likelihoods transfer over.
     //
-    assert(!target->IsLast());
-    BasicBlock* next = fgNewBBafter(BBJ_ALWAYS, block, true, target->GetFalseTarget());
-
-    // Fix up block's flow
-    //
-    block->SetCond(target->GetTrueTarget(), next);
-    fgAddRefPred(block->GetTrueTarget(), block);
     fgRemoveRefPred(target, block);
 
-    // The new block 'next' will inherit its weight from 'block'
-    //
-    next->inheritWeight(block);
-    fgAddRefPred(next, block);
-    fgAddRefPred(next->GetTarget(), next);
+    FlowEdge* const targetTrueEdge  = fgGetPredForBlock(target->GetTrueTarget(), target);
+    FlowEdge* const targetFalseEdge = fgGetPredForBlock(target->GetFalseTarget(), target);
+    block->SetCond(target->GetTrueTarget(), target->GetFalseTarget());
+    fgAddRefPred(block->GetTrueTarget(), block, targetTrueEdge);
+    fgAddRefPred(block->GetFalseTarget(), block, targetFalseEdge);
 
-    JITDUMP("fgOptimizeUncondBranchToSimpleCond(from " FMT_BB " to cond " FMT_BB "), created new uncond " FMT_BB "\n",
-            block->bbNum, target->bbNum, next->bbNum);
+    JITDUMP("fgOptimizeUncondBranchToSimpleCond(from " FMT_BB " to cond " FMT_BB "), modified " FMT_BB "\n",
+            block->bbNum, target->bbNum, block->bbNum);
     JITDUMP("   expecting opts to key off V%02u in " FMT_BB "\n", lclNum, block->bbNum);
 
+    if (target->hasProfileWeight() && block->hasProfileWeight())
+    {
+        // Remove weight from target since block now bypasses it...
+        //
+        weight_t targetWeight = target->bbWeight;
+        weight_t blockWeight  = block->bbWeight;
+        target->setBBProfileWeight(max(0, targetWeight - blockWeight));
+        JITDUMP("Decreased " FMT_BB " profile weight from " FMT_WT " to " FMT_WT "\n", target->bbNum, targetWeight,
+                target->bbWeight);
+    }
+
     return true;
 }
 
@@ -4817,13 +4822,11 @@ bool Compiler::fgUpdateFlowGraph(bool doTailDuplication /* = false */, bool isPh
                 if (doTailDuplication && fgOptimizeUncondBranchToSimpleCond(block, bDest))
                 {
                     assert(block->KindIs(BBJ_COND));
-                    change   = true;
-                    modified = true;
-                    bDest    = block->GetTrueTarget();
-                    bNext    = block->GetFalseTarget();
-
-                    // TODO-NoFallThrough: Adjust the above logic once bbFalseTarget can diverge from bbNext
-                    assert(block->NextIs(bNext));
+                    assert(bNext == block->Next());
+                    change     = true;
+                    modified   = true;
+                    bDest      = block->GetTrueTarget();
+                    bFalseDest = block->GetFalseTarget();
                 }
             }