intel/nir: Move 64-bit lowering later

intel/nir: Move 64-bit lowering later

Now that we have a loop unrolling cost function and loop unrolling isn't
going to kill us the moment we have a 64-bit op in a loop, we can go
ahead and move 64-bit lowering later.  This gives us the opportunity to
do more optimizations and actually let the full optimizer run even on
64-bit ops rather than hoping one round of opt_algebraic will fix
everything.  This substantially reduces both fp64 shader compile times
and the resulting code size.  On the vs-isnan-dvec test from piglit:

Before this commit:

    1684.63s user 17.29s system 99% cpu 28:28.24 total
    101479 instructions. 0 loops. 802452 cycles. 79:369 spills:fills.
    Peak memory usage (according to massif): 1.435 GB

After this commit:

    179.64s user 7.75s system 99% cpu 3:07.92 total
    57316 instructions. 0 loops. 459287 cycles. 0:0 spills:fills.
    Peak memory usage (according to massif): 531.0 MB

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