In Section 12.30 of the ESSL 3.00 spec on p143-144:

  Indexing of arrays of samplers by constant-index-expressions is
  supported in GLSL ES 1.00. A constant-index-expression is an
  expression formed from constant-expressions and certain loop indices,
  defined for a subset of loop constructs. Should this functionality be
  included in GLSL ES 3.00?

  RESOLUTION: No. Arrays of samplers may only be indexed by constant-
  integral-expressions.

Bug: chromium:985032
Change-Id: Iad9e4b8000b442a6631e1d2295c2670c4a79d486
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1849452
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>