glsl: Permit non-ubo input interface arrays to use non-const indexing.

glsl: Permit non-ubo input interface arrays to use non-const indexing.

From the GLSL ES 3.00 spec:

    "All indexes used to index a uniform block array must be constant
    integral expressions."

Similar text exists in GLSL specs since 1.50.

When we implemented this, the only type of interface block supported
by Mesa was uniform blocks, so we required all indexes used to index
any interface block to be constant integral expressions.

Now that we are adding interface block support for GLSL 1.50, we need
a more specific check.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>