Until now, we had separate passes for lowering gl_PatchVerticesIn to
a statically known constant (for TES inputs when linked against a TCS),
and a uniform in the other cases.  Annoyingly, one had to be run before
nir_lower_system_values, and the other afterward.  This simplified the
passes, but made life painful for the callers.

This patch combines both into a single pass.  If you give it a non-zero
static count, it uses that.  If you give it Mesa state slots, it turns
it back into a built-in uniform.  Otherwise, it does nothing.

This also moves the i965 uniform lowering out to shared code.

v2: Make token arrays const.

Reviewed-by: Eric Anholt <eric@anholt.net>