nir/sink: Rewrite loop handling logic

Previously, for code like:
loop {
    loop {
        a = load_ubo()
    }
    use(a)
}
adjust_block_for_loops() would return the block before the first loop.
Now we compute the range of allowed blocks and then walk the dominance
tree directly, guaranteeing directly that we always choose a block that
dominates all the uses and is dominated by the definition.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
(cherry picked from commit af9296b8)