Take advantage of the recent improvements to the liveintervals set (tracking

Take advantage of the recent improvements to the liveintervals set (tracking
instructions which define each value#) to simplify and improve the coallescer.
In particular, this patch:

1. Implements iterative coallescing.
2. Reverts an unsafe hack from handlePhysRegDef, superceeding it with a
   better solution.
3. Implements PR865, "coallescing" away the second copy in code like:

   A = B
   ...
   B = A

This also includes changes to symbolically print registers in intervals
when possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29862 91177308-0d34-0410-b5e6-96231b3b80d8