[RegAllocGreedy] Provide a subtarget hook to disable the local reassignment

[RegAllocGreedy] Provide a subtarget hook to disable the local reassignment
heuristic.
By default, no functionality change.
This is a follow-up of r212099.

This hook provides a finer grain to control the optimization.

<rdar://problem/17444599>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212204 91177308-0d34-0410-b5e6-96231b3b80d8