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

Test/hlsl.struct.frag

@@ -12,7 +12,7 @@ struct myS {
 
 myS s1;
 
-struct {
+static struct {
     float4 i;
 } s2;
 
@@ -37,7 +37,7 @@ float4 PixelShaderFunction(float4 input, IN_S s) : COLOR0
     } s3;
 
     s3 == s3;
-    s2.i; s.ff4; // no assignments to uniforms, but preserve indirections.
+    s2.i = s.ff4;
 
     return input;
 }