ir3/ra: Fix tied destination handling with multiple destinations

ir3/ra: Fix tied destination handling with multiple destinations

Before, we were careful to

1. Get the source physreg.
2. Allocate the destination.
3. Insert a copy with the source being the physreg from step 1.

and this guaranteed that if the tied source were moved in step 2 we'd
still insert a copy from the correct place. However this won't work with
multiple destinations because an earlier destination could've already
moved the tied source around. Instead flip steps 2 and 3 (we'll insert
the copy before we allocate the interval, but that's ok) and run the
first two steps in a separate loop before any destinations are
allocated.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14107>