nir/validate: Allow subroutine types for the tails of derefs

nir/validate: Allow subroutine types for the tails of derefs

The shader-subroutine code creates uniforms of type SUBROUTINE for
subroutines that are then read as integers in the backends.  If we ever
want to do any optimizations on these, we'll need to come up with a better
plan where they are actual scalars or something, but this works for now.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92859
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>