Non-integer indices are already rejected in the parser, so the
ValidateLimitations pass doesn't need to check for them.

ESSL 1.00 spec is not actually terribly clear about whether the
parser should do this check, but the language grammar in the spec only
has indexing with "integer_expression" so it seems like ANGLE's
interpretation of only allowing indexing with integers is correct.
ESSL 3.00 makes this restriction explicitly clear in section 5.7.

BUG=angleproject:1254
TEST=angle_unittests

Change-Id: I02b2a6f4d9fa7801a98df63ed21bc990e1585eb8
Reviewed-on: https://chromium-review.googlesource.com/317741Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>