nir: Release per-block metadata in nir_sweep

nir: Release per-block metadata in nir_sweep

nir_sweep already marks all metadata invalid, so it is safe to release
the memory here too.

mean soft fp64 using uint64:   1,342,759,331 => 1,010,670,475
gfxbench5 aztec ruins high 11:    63,555,571 =>    61,889,811
deus ex mankind divided 148:      62,845,304 =>    62,829,640
deus ex mankind divided 2890:     71,922,686 =>    71,922,686
dirt showdown 676:                69,238,607 =>    69,238,607
dolphin ubershaders 210:          77,822,072 =>    77,822,072

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>