Implement RAGreedy::splitAroundRegion and remove loop splitting.
Region splitting includes loop splitting as a subset, and it is more generic.
The splitting heuristics for variables that are live in more than one block are
now:
1. Try to create a region that covers multiple basic blocks.
2. Try to create a new live range for each block with multiple uses.
3. Spill.
Steps 2 and 3 are similar to what the standard spiller is doing.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123853
91177308-0d34-0410-b5e6-
96231b3b80d8