glsl: remove gl_nir_opt_dead_builtin_varyings

It doesn't do anything since IO variables are lowered to intrinsics,
which simplifies and eliminates a lot of variable-specific stuff
like declared but dead builtin varyings and unused components
of builtin varying arrays.

This reduces GLSL compile times by 2.4% with the gallium noop driver.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Timothy Arceri <tarceri@itqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36023>
4 files changed