Array constructors that are used as a statement by themselves need to
be pruned from the AST before writing HLSL output.

This fixes an assert in OutputHLSL.

BUG=angleproject:2307
TEST=angle_unittests

Change-Id: Ib49461a4be173f3856f5a264ac0af8d818a61798
Reviewed-on: https://chromium-review.googlesource.com/874691
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>