Implement extension GL_AMD_gpu_shader_int16

- Add int16 types (int16_t, uint16_t, i16vec, u16vec).
- Add int16 support to GLSL operators.
- Add int16 type conversions (to int16, from int16).
- Add int16 built-in functions.

glslang/MachineIndependent/linkValidate.cpp

@@ -1057,6 +1057,8 @@ int TIntermediate::getBaseAlignmentScalar(const TType& type, int& size)
     case EbtUint64:
     case EbtDouble:  size = 8; return 8;
 #ifdef AMD_EXTENSIONS
+    case EbtInt16:
+    case EbtUint16:
     case EbtFloat16: size = 2; return 2;
 #endif
     default:         size = 4; return 4;