[turbofan] Clean up and factor out branch generation logic.

src/compiler/instruction-selector-impl.h

Index: src/compiler/instruction-selector-impl.h
diff --git a/src/compiler/instruction-selector-impl.h b/src/compiler/instruction-selector-impl.h
index 53e288df3e19a705694394d02625b269327a04c1..5e4c09055e46a3ffe0aba26da538d5b1ff843765 100644
--- a/src/compiler/instruction-selector-impl.h
+++ b/src/compiler/instruction-selector-impl.h
@@ -257,7 +257,7 @@ class FlagsContinuation FINAL {
 
   void Negate() {
     DCHECK(!IsNone());
-    condition_ = static_cast<FlagsCondition>(condition_ ^ 1);
+    condition_ = NegateFlagsCondition(condition_);
   }
 
   void Commute() {
@@ -317,8 +317,6 @@ class FlagsContinuation FINAL {
     if (negate) Negate();
   }
 
-  void SwapBlocks() { std::swap(true_block_, false_block_); }
-
   // Encodes this flags continuation into the given opcode.
   InstructionCode Encode(InstructionCode opcode) {
     opcode |= FlagsModeField::encode(mode_);