nir: Teach loop unrolling about 64-bit instruction lowering

The lowering we do for 64-bit instructions can cause a single NIR ALU
instruction to blow up into hundreds or thousands of instructions
potentially with control flow.  If loop unrolling isn't aware of this,
it can unroll a loop 20 times which contains a nir_op_fsqrt which we
then lower to a full software implementation based on integer math.
Those 20 invocations suddenly get a lot more expensive than NIR loop
unrolling currently expects.  By giving it an approximate estimate
function, we can prevent loop unrolling from going to town when it
shouldn't.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_opt_loop_unroll.c

@@ -564,7 +564,7 @@ is_loop_small_enough_to_unroll(nir_shader *shader, nir_loop_info *li)
       return true;
 
    bool loop_not_too_large =
-      li->num_instructions * li->max_trip_count <= max_iter * LOOP_UNROLL_LIMIT;
+      li->instr_cost * li->max_trip_count <= max_iter * LOOP_UNROLL_LIMIT;
 
    return loop_not_too_large;
 }