Make the coallescer a bit smarter, allowing it to join more live ranges.

Make the coallescer a bit smarter, allowing it to join more live ranges.
For example, we can now join things like [0-30:0)[31-40:1)[52-59:2)
with [40:60:0) if the 52-59 range is defined by a copy from the 40-60 range.
The resultant range ends up being [0-30:0)[31-60:1).

This fires a lot through-out the test suite (e.g. shrinking bc from
19492 -> 18509 machineinstrs) though most gains are smaller (e.g. about
50 copies eliminated from crafty).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23866 91177308-0d34-0410-b5e6-96231b3b80d8