Fold user-definedness of function nodes into TOperator

Whether a function call is user-defined is not orthogonal to TOperator
associated with the call node - other ops than function calls can't be
user-defined. Because of this it makes sense to store the user-
definedness by having different TOperator enums for different types of
calls.

This patch also tags internal helper functions that have a raw
definition outside the AST with a separate TOperator enum. This way
they can be handled with logic that is easy to understand. Before this,
function calls like this left the user-defined bit unset, despite not
really being built-ins either. The EmulatePrecision traverser uses
this. This is also something that could be used to clean up built-in
emulation in the future.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I597fcd9789d0cc22b689ef3ce5a0cc3f621d4859
Reviewed-on: https://chromium-review.googlesource.com/433443
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/ValidateGlobalInitializer.cpp b/src/compiler/translator/ValidateGlobalInitializer.cpp
index c4caa29..10a3e51 100644
--- a/src/compiler/translator/ValidateGlobalInitializer.cpp
+++ b/src/compiler/translator/ValidateGlobalInitializer.cpp
@@ -72,8 +72,8 @@
     // Disallow calls to user-defined functions and texture lookup functions in global variable
     // initializers.
     // This is done simply by disabling all function calls - built-in math functions don't use
-    // EOpFunctionCall.
-    if (node->getOp() == EOpFunctionCall)
+    // the function call ops.
+    if (node->isFunctionCall())
     {
         mIsValid = false;
     }