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

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