Two changes here:

  1. Instead of copying Local graphs to the BU graphs to start with, use
     spliceFrom to do the job (which is constant time in this case).  On
     176.gcc, this chops off .17s from the bu pass.
  2. When building SCC graphs, simplify the logic and use spliceFrom to
     do the heavy lifting, instead of cloneInto/delete.  This slices
     another .14s off 176.gcc.

llvm-svn: 20826