mesa: Add a new function for getting the nonconst sampler array index

mesa: Add a new function for getting the nonconst sampler array index

If the array index is not a constant expression, the existing support
will assume a zero offset (giving us the sampler index of the base of
the array).

For dynamically uniform indexing of sampler arrays, we need both that
and the indexing expression.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>