Rename live range stages to better reflect how they are used.

The stage is used to control where a live range is going, not where it
is coming from. Live ranges created by splitting will usually be marked
RS_New, but some are marked RS_Spill to avoid wasting time trying to
split them again.

The old RS_Global and RS_Local stages are merged - they are really the
same thing for local and global live ranges.

llvm-svn: 135911