[turbofan] Fold "boolean not"-like Selects into branches.

Fold a Select that negates a boolean value, i.e. returning true in the
false case and vice versa, into Branch users, similar to what we already
do for Branch nodes with BooleanNot inputs.

BUG=v8:5267

Review-Url: https://codereview.chromium.org/2308303003
Cr-Commit-Position: refs/heads/master@{#39149}

src/compiler/common-operator-reducer.cc

@@ -77,8 +77,12 @@ Reduction CommonOperatorReducer::ReduceBranch(Node* node) {
   // Swap IfTrue/IfFalse on {branch} if {cond} is a BooleanNot and use the input
   // to BooleanNot as new condition for {branch}. Note we assume that {cond} was
   // already properly optimized before we get here (as guaranteed by the graph
-  // reduction logic).
-  if (cond->opcode() == IrOpcode::kBooleanNot) {
+  // reduction logic). The same applies if {cond} is a Select acting as boolean
+  // not (i.e. true being returned in the false case and vice versa).
+  if (cond->opcode() == IrOpcode::kBooleanNot ||
+      (cond->opcode() == IrOpcode::kSelect &&
+       DecideCondition(cond->InputAt(1)) == Decision::kFalse &&
+       DecideCondition(cond->InputAt(2)) == Decision::kTrue)) {
     for (Node* const use : node->uses()) {
       switch (use->opcode()) {
         case IrOpcode::kIfTrue:

test/unittests/compiler/common-operator-reducer-unittest.cc

@@ -158,6 +158,26 @@ TEST_F(CommonOperatorReducerTest, BranchWithBooleanNot) {
   }
 }
 
+TEST_F(CommonOperatorReducerTest, BranchWithSelect) {
+  Node* const value = Parameter(0);
+  TRACED_FOREACH(BranchHint, hint, kBranchHints) {
+    Node* const control = graph()->start();
+    Node* const branch = graph()->NewNode(
+        common()->Branch(hint),
+        graph()->NewNode(common()->Select(MachineRepresentation::kTagged),
+                         value, FalseConstant(), TrueConstant()),
+        control);
+    Node* const if_true = graph()->NewNode(common()->IfTrue(), branch);
+    Node* const if_false = graph()->NewNode(common()->IfFalse(), branch);
+    Reduction const r = Reduce(branch);
+    ASSERT_TRUE(r.Changed());
+    EXPECT_EQ(branch, r.replacement());
+    EXPECT_THAT(branch, IsBranch(value, control));
+    EXPECT_THAT(if_false, IsIfTrue(branch));
+    EXPECT_THAT(if_true, IsIfFalse(branch));
+    EXPECT_EQ(NegateBranchHint(hint), BranchHintOf(branch->op()));
+  }
+}
 
 // -----------------------------------------------------------------------------
 // Merge