Allow non-trivial constant expressions for array sizes.

This fixes GLSL ES2 conformance test `array`.

Change-Id: I6ebee9253e1e8c394d9ddb6899e3a0940b7a38ef
Bug: skia:12495
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/458718
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
7 files changed