nir/linker: use nir_gather_xfb_info

nir/linker: use nir_gather_xfb_info

Instead of a custom ARB_gl_spirv xfb gather info pass.

In fact, this is not only about reusing code, but the current custom
code was not handling properly how many varyings are enumerated from
some complex types. So this change is also about fixing some corner
cases.

v2: Use util_bitcount, simplify current stage check (Kenneth)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>