nir/gcm: Rework the schedule late loop

nir/gcm: Rework the schedule late loop

This fixes a bug in code motion that occurred when the best block is the
same as the schedule early block.  In this case, because we're checking
(lca != def->parent_instr->block) at the top of the loop, we never get to
the check for loop depth so we wouldn't move it out of the loop.  This
commit reworks the loop to be a simple for loop up the dominator chain and
we place the (lca != def->parent_instr->block) check at the end of the
loop.

Reviewed-by: Matt Turner <mattst88@gmail.com>