Optimize the result of a conditional break/continue. In NIR something
like:

loop {
   ...
   if (cond)
      continue;

would get lowered to:

block_0:
...
block_1:
branch_cond !cond block_3
block_2:
branch_uncond block_0
block_3:
...

We recognize the conditional branch skipping over the unconditional
branch, and turn it into:

block_0:
...
block_1:
branch_cond cond block_0
block_2:
block_3:

Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <mesa/mesa!4125>