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 <[email protected]>
Reviewed-by: Jordan Justen <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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