Add IR dumping functions to use during debugging.

When using lldb and/or gdb I frequently get odd std::string failures
when using the IR printing instructions we have now.  This adds the
methods  Instruction::Dump(), BasicBlock::Dump() and Function::Dump() to
emit the output of the pretty print to stderr.

With this I can now reliably print IR from gdb and lldb sessions.

source/opt/basic_block.cpp

@@ -193,6 +193,10 @@ std::ostream& operator<<(std::ostream& str, const BasicBlock& block) {
   return str;
 }
 
+void BasicBlock::Dump() const {
+  std::cerr << "Basic block #" << id() << "\n" << *this << "\n ";
+}
+
 std::string BasicBlock::PrettyPrint(uint32_t options) const {
   std::ostringstream str;
   ForEachInst([&str, options](const Instruction* inst) {

source/opt/basic_block.h

@@ -209,6 +209,10 @@ class BasicBlock {
   // is always added to |options|.
   std::string PrettyPrint(uint32_t options = 0u) const;
 
+  // Dump this basic block on stderr.  Useful when running interactive
+  // debuggers.
+  void Dump() const;
+
  private:
   // The enclosing function.
   Function* function_;

source/opt/function.cpp

@@ -94,6 +94,10 @@ std::ostream& operator<<(std::ostream& str, const Function& func) {
   return str;
 }
 
+void Function::Dump() const {
+  std::cerr << "Function #" << result_id() << "\n" << *this << "\n";
+}
+
 std::string Function::PrettyPrint(uint32_t options) const {
   std::ostringstream str;
   ForEachInst([&str, options](const Instruction* inst) {

source/opt/function.h

@@ -125,6 +125,10 @@ class Function {
   // is always added to |options|.
   std::string PrettyPrint(uint32_t options = 0u) const;
 
+  // Dump this function on stderr.  Useful when running interactive
+  // debuggers.
+  void Dump() const;
+
  private:
   // The OpFunction instruction that begins the definition of this function.
   std::unique_ptr<Instruction> def_inst_;

source/opt/instruction.cpp

@@ -540,6 +540,10 @@ std::ostream& operator<<(std::ostream& str, const Instruction& inst) {
   return str;
 }
 
+void Instruction::Dump() const {
+  std::cerr << "Instruction #" << unique_id() << "\n" << *this << "\n";
+}
+
 bool Instruction::IsOpcodeCodeMotionSafe() const {
   switch (opcode_) {
     case SpvOpNop:

source/opt/instruction.h

@@ -427,6 +427,10 @@ class Instruction : public utils::IntrusiveNodeBase<Instruction> {
   // rules for physical addressing.
   bool IsValidBasePointer() const;
 
+  // Dump this instruction on stderr.  Useful when running interactive
+  // debuggers.
+  void Dump() const;
+
  private:
   // Returns the total count of result type id and result id.
   uint32_t TypeResultIdCount() const {