[BOLT] Add BinaryFunction::registerBranch(). NFC (#83337)

Add an external interface to register a branch in a function that is in
disassembled state. Allows to make custom modifications to the
disassembler. E.g., a pre-CFG pass can add an instruction and register a
branch that will later be used during the CFG construction.

bolt/include/bolt/Core/BinaryFunction.h

@@ -2056,6 +2056,14 @@ public:
   /// Returns false if disassembly failed.
   Error disassemble();
 
+  /// An external interface to register a branch while the function is in
+  /// disassembled state. Allows to make custom modifications to the
+  /// disassembler. E.g., a pre-CFG pass can add an instruction and register
+  /// a branch that will later be used during the CFG construction.
+  ///
+  /// Return a label at the branch destination.
+  MCSymbol *registerBranch(uint64_t Src, uint64_t Dst);
+
   Error handlePCRelOperand(MCInst &Instruction, uint64_t Address,
                            uint64_t Size);
 

bolt/lib/Core/BinaryFunction.cpp

@@ -1445,6 +1445,16 @@ add_instruction:
   return Error::success();
 }
 
+MCSymbol *BinaryFunction::registerBranch(uint64_t Src, uint64_t Dst) {
+  assert(CurrentState == State::Disassembled &&
+         "Cannot register branch unless function is in disassembled state.");
+  assert(containsAddress(Src) && containsAddress(Dst) &&
+         "Cannot register external branch.");
+  MCSymbol *Target = getOrCreateLocalLabel(Dst);
+  TakenBranches.emplace_back(Src - getAddress(), Dst - getAddress());
+  return Target;
+}
+
 bool BinaryFunction::scanExternalRefs() {
   bool Success = true;
   bool DisassemblyFailed = false;