nir: clone instruction set rather than removing individual entries

This reduces the time spent in nir_opt_cse() by almost a half.

The massif tool from callgrind reported no change in peak
memory use with the large doliphin uber shaders I used for
testing.

Reviewed-by: Thomas Helland<thomashelland90@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
1 file changed