Simplify the eviction policy by making the failsafe explicit.

When assigned ranges are evicted, they are put in the RS_Evicted stage and are
not allowed to evict anything else. That prevents looping automatically.

When evicting ranges just to get a cheaper register, use only spill weights to
find the possible candidates. Avoid breaking hints for this purpose, it is not
worth it.

Start implementing more complex eviction heuristics, guarded by the temporary
-complex-eviction flag. The initial version permits a heavier range to be
evicted if it doesn't have any uses where the evicting range is live. This makes
it a good candidate for live ranfge splitting.

llvm-svn: 132358