nir/cf: add new control modification API's

nir/cf: add new control modification API's

These will help us do a number of things, including:

- Early return elimination.
- Dead control flow elimination.
- Various optimizations, such as replacing:

if (foo) {
    ...
}
if (!foo) {
    ...
}

with:

if (foo) {
    ...
} else {
    ...
}

Signed-off-by: Connor Abbott <connor.w.abbott@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>