nir: Look into uniform structs for samplers when counting num_textures.

nir: Look into uniform structs for samplers when counting num_textures.

mesa/st decides whether to update samplers after a program change based on
whether num_textures is nonzero.  By not counting samplers in a uniform
struct, we would segfault in
KHR-GLES3.shaders.struct.uniform.sampler_vertex if it was run in the same
context after a non-vertex-shader-uniform testcase (as is the case during
a full conformance run).

v2: Implement using two separate pure functions instead of updating
    pointers.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>