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

src/compiler/instruction-codes.h

Index: src/compiler/instruction-codes.h
diff --git a/src/compiler/instruction-codes.h b/src/compiler/instruction-codes.h
index 00fa1b4b1dc9465ba588c96af8f478bfeddacce1..21a0f788143dfb5865ee86b00e0982542091fc50 100644
--- a/src/compiler/instruction-codes.h
+++ b/src/compiler/instruction-codes.h
@@ -98,6 +98,10 @@ enum FlagsCondition {
   kNotOverflow
 };
 
+inline FlagsCondition NegateFlagsCondition(FlagsCondition condition) {
+  return static_cast<FlagsCondition>(condition ^ 1);
+}
+
 std::ostream& operator<<(std::ostream& os, const FlagsCondition& fc);
 
 // The InstructionCode is an opaque, target-specific integer that encodes