glsl: validate sampler array indexing for 'constant-index-expression'

glsl: validate sampler array indexing for 'constant-index-expression'

Desktop GLSL < 130 and GLSL ES < 300 allow sampler array indexing where
index can contain a loop induction variable. This extra check will warn
during linking if some of the indexes could not be turned in to constant
expressions.

v2: warning instead of error for backends that did not enable
    EmitNoIndirectSampler option (have dynamic indexing)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Cc: "10.5" and "10.6" <mesa-stable@lists.freedesktop.org>