AMDGPU/GlobalISel: Set insert point after waterfall loop

AMDGPU/GlobalISel: Set insert point after waterfall loop

The current users of the waterfall loop utility functions do not make
use of the restored original insert point. The insertion is either
done, or they set the insert point somewhere else. A future change
will want to insert instructions after the waterfall loop, but
figuring out the point after the loop is more difficult than ensuring
the insert point is there after the loop.