Register Allocator: refactoring (no functionality change).

Factor out two functions calculateRegionSplitCost and doRegionSplit
from tryRegionSplit. These two functions will be used in coming patches.

rdar://16162005


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204684 91177308-0d34-0410-b5e6-96231b3b80d8