nir: Strengthen "no jumps" assertions in instruction insertion API.

nir: Strengthen "no jumps" assertions in instruction insertion API.

Jumps must be the last instruction in a block, so inserting another
instruction after a jump is illegal.

Previously, we only checked this when the new instruction being inserted
was a jump.  This is a red herring - inserting *any* kind of instruction
after a jump is illegal.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Acked-by: Connor Abbott <cwabbott0@gmail.com>