gallivm: increase nesting limit to 66

gallivm: increase nesting limit to 66

This is still not really correct, since at least for sm 4.0
the nesting limit is 64 per subroutine, and subroutine nesting itself
has a limit of 32, so since we have a flat stack we'd need 32*64.
But this should probably be better fixed with per-subroutine stacks,
since otherwise these structures get really big (like 100kB for the
lp_exec_mask).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>