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/265654Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>