Clean up function name mangling code

Fix a few incorrect comments about mangled names, and refactor
generating mangled names from function call nodes.

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: I3ee68c4c0982f1a9c28d8e87aafa19f19559bbf8
Reviewed-on: https://chromium-review.googlesource.com/465826
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/compiler/translator/IntermTraverse.cpp b/src/compiler/translator/IntermTraverse.cpp
index 552979d..1819ab8 100644
--- a/src/compiler/translator/IntermTraverse.cpp
+++ b/src/compiler/translator/IntermTraverse.cpp
@@ -690,7 +690,8 @@
             TFunction *builtInFunc = nullptr;
             if (!node->isFunctionCall() && !node->isConstructor())
             {
-                builtInFunc = mSymbolTable.findBuiltInOp(node, mShaderVersion);
+                builtInFunc = static_cast<TFunction *>(
+                    mSymbolTable.findBuiltIn(node->getSymbolTableMangledName(), mShaderVersion));
             }
 
             size_t paramIndex = 0;