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/operand.cpp b/source/operand.cpp
index 89912e0..91814b0 100644
--- a/source/operand.cpp
+++ b/source/operand.cpp
@@ -1454,6 +1454,8 @@
       return "literal";
     case SPV_OPERAND_TYPE_LITERAL_NUMBER:
       return "literal number";
+    case SPV_OPERAND_TYPE_MULTIWORD_LITERAL_NUMBER:
+      return "multiple word literal number";
     case SPV_OPERAND_TYPE_LITERAL_STRING:
       return "literal string";
     case SPV_OPERAND_TYPE_SOURCE_LANGUAGE: