Add an AST traverser that rejects shaders with expressions that use the return
value of an assignment where an array is assigned to another array. In the
future this should be made into a tool that can simplify these expressions so
that return values of array assignments are not used.

In its current form this code prevents OutputHLSL from producing garbage code
when the original code contains expressions like a = (b = c); where a, b, and c
are all arrays.

BUG=angleproject:960

Change-Id: I11353d7ed7160c853e58a0ef3471ca439cb314c8
Reviewed-on: https://chromium-review.googlesource.com/263070Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>