HLSL: phase 2b: add l-value operator[] for RWTexture/RWBuffer

HLSL: phase 2b: add l-value operator[] for RWTexture/RWBuffer

This commit adds l-value support for RW texture and buffer objects.
Supported are:

- pre and post inc/decrement
- function out parameters
- op-assignments, such as *=, +-, etc.
- result values from op-assignments.  e.g, val=(MyRwTex[loc] *= 2);

Not supported are:
- Function inout parameters
- multiple post-inc/decrement operators.  E.g, MyRWTex[loc]++++;