nir/opt_if: fix opt_if_merge when destination branch has a jump
Fixes a case where opt_if_merge created code like:
if (...) {
break;
loop {
...
}
}
which caused opt_peel_loop_initial_if to complain that the loop pre-header wasn't a predecessor of the loop header. This patch prevents this (invalid, I think) unreachable code from being created.