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/sampler-struct-bug 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