Support multiple word literal numbers as constants.

Add a new operand type SPV_OPERAND_TYPE_MULTIWORD_LITERAL_NUMBER
to represent those operands that can expand into multiple words.
Now only OpConstant and OpSpecConstant have such kind of operand.
diff --git a/source/opcode.cpp b/source/opcode.cpp
index 4507da6..8701eaf 100644
--- a/source/opcode.cpp
+++ b/source/opcode.cpp
@@ -90,6 +90,9 @@
       default:
         break;
     }
+  } else if (operandClass == OperandVariableLiterals) {
+    if (opcode == spv::OpConstant || opcode == spv::OpSpecConstant)
+      return SPV_OPERAND_TYPE_MULTIWORD_LITERAL_NUMBER;
   }
 
   switch(operandClass) {