Added RegisterCoalescer support for joining global copies first.

This adds the -join-globalcopies option which can be enabled by
default once misched is also enabled.

Ideally, the register coalescer would be able to split local live
ranges in a way that produces copies that can be easily resolved by
the scheduler. Until then, this heuristic should be good enough to at
least allow the scheduler to run after coalescing.

llvm-svn: 167825