* If there's one usage of a in the block of b, then
* we check, if this use is dominated by b, if that's true
* a and b interfere. Note that b must strictly dominate the user,
* If there's one usage of a in the block of b, then
* we check, if this use is dominated by b, if that's true
* a and b interfere. Note that b must strictly dominate the user,