glsl: Set location on structure-split sampler uniform variables

gl_nir_lower_samplers_as_deref splits structure uniform variables,
creating new variables for individual fields.  As part of that, it
calculates a new location.  It then never set this on the new variables.

Thanks to Michael Fiano for finding this bug.  Fixes crashes on i965
with Piglit's new tests/spec/glsl-1.10/execution/samplers/uniform-struct
test, which was reduced from the failing case in Michael's app.

Fixes: f003859f nir: Make gl_nir_lower_samplers use gl_nir_lower_samplers_as_deref
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 9e0c744f)