Add basic support for the length() method of arrays Support expressions where the expression that .length() is called on does not have side effects. Tested with WebGL 2 test sdk/tests/deqp/data/gles3/shaders/arrays.html TEST=WebGL 2 conformance tests BUG=angleproject:972 Change-Id: Ib4f8377a51da61179b6e47fbcf6b4d915e351fbd Reviewed-on: https://chromium-review.googlesource.com/265654 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com>