Add opt::Operand::AsCString and AsString (#3240)

It only works when the operand is a literal string.

source/opt/instruction.h

@@ -80,6 +80,15 @@ struct Operand {
   spv_operand_type_t type;  // Type of this logical operand.
   OperandData words;        // Binary segments of this logical operand.
 
+  // Returns a string operand as a C-style string.
+  const char* AsCString() const {
+    assert(type == SPV_OPERAND_TYPE_LITERAL_STRING);
+    return reinterpret_cast<const char*>(words.data());
+  }
+
+  // Returns a string operand as a std::string.
+  std::string AsString() const { return AsCString(); }
+
   friend bool operator==(const Operand& o1, const Operand& o2) {
     return o1.type == o2.type && o1.words == o2.words;
   }

test/opt/instruction_test.cpp

@@ -14,6 +14,7 @@
 
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "gmock/gmock.h"
@@ -60,6 +61,18 @@ TEST(InstructionTest, CreateWithOpcodeAndNoOperands) {
   EXPECT_EQ(inst.end(), inst.begin());
 }
 
+TEST(InstructionTest, OperandAsCString) {
+  Operand::OperandData abcde{0x64636261, 0x65};
+  Operand operand(SPV_OPERAND_TYPE_LITERAL_STRING, std::move(abcde));
+  EXPECT_STREQ("abcde", operand.AsCString());
+}
+
+TEST(InstructionTest, OperandAsString) {
+  Operand::OperandData abcde{0x64636261, 0x65};
+  Operand operand(SPV_OPERAND_TYPE_LITERAL_STRING, std::move(abcde));
+  EXPECT_EQ("abcde", operand.AsString());
+}
+
 // The words for an OpTypeInt for 32-bit signed integer resulting in Id 44.
 uint32_t kSampleInstructionWords[] = {(4 << 16) | uint32_t(SpvOpTypeInt), 44,
                                       32, 1};