nir: Fix holes in nir_instr

nir: Fix holes in nir_instr

Found using pahole.

Changes in peak memory usage according to Valgrind massif:

mean soft fp64 using uint64:   1,343,991,403 => 1,342,759,331
gfxbench5 aztec ruins high 11:    63,619,971 =>    63,555,571
deus ex mankind divided 148:      62,887,728 =>    62,845,304
deus ex mankind divided 2890:     72,399,750 =>    71,922,686
dirt showdown 676:                69,464,023 =>    69,238,607
dolphin ubershaders 210:          78,359,728 =>    77,822,072

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