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.h

@@ -1915,8 +1915,8 @@ typedef struct {
 } nir_loop_terminator;
 
 typedef struct {
-   /* Number of instructions in the loop */
-   unsigned num_instructions;
+   /* Estimated cost (in number of instructions) of the loop */
+   unsigned instr_cost;
 
    /* Maximum number of times the loop is run (if known) */
    unsigned max_trip_count;