HLSL: phase 2e: introduce lower level addBinaryNode/UnaryNode fns

- hlsl.struct.frag variable changed to static, assignment replacd.

- Created new low level functions addBinaryNode and addUnaryNode.  These are
  used by higher level functions such as addAssignment, and do not do any
  argument promotion or conversion of any sort.

- Two functions above are now used in RWTexture lvalue conversions.  Also,
  other direction creations of unary or binary nodes now use them, e.g, addIndex.
  This cleans up some existing code.

- removed handling of EOpVectorTimesScalar from promote()

- removed comment from ParseHelper.cpp

glslang/MachineIndependent/ParseHelper.cpp

@@ -1966,7 +1966,6 @@ bool TParseContext::lValueErrorCheck(const TSourceLoc& loc, const char* op, TInt
     if (TParseContextBase::lValueErrorCheck(loc, op, node))
         return true;
 
-    // GLSL specific
     const char* symbol = nullptr;
     TIntermSymbol* symNode = node->getAsSymbolNode();
     if (symNode != nullptr)