nir/search: Fold src_is_bool()/alu_instr_is_bool() into src_is_type().

nir/search: Fold src_is_bool()/alu_instr_is_bool() into src_is_type().

I don't want src_is_bool() and src_is_type(x, nir_type_bool) to behave
differently.  Having the logic spread out over three functions makes it
harder to decide where to put new logic, as well.

So, combine them all.  It's a bit simpler because there's now only one
recursive function rather than a pair of mutually recursive functions.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>