nir: fix sampler lowering pass for arrays

nir: fix sampler lowering pass for arrays

This fixes bugs with special cases where we have arrays of
structures containing samplers or arrays of samplers.

I've verified that patch results in calculating same index value as
returned by _mesa_get_sampler_uniform_value for IR. Patch makes
following ES3 conformance test pass:

	ES3-CTS.shaders.struct.uniform.sampler_array_fragment

v2: remove unnecessary comment (Topi)
    simplify changes and the overall code (Jason)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90114