Assert that the target provided hints are in the allocation order.

Target implementations of getRegAllocationHints() should use the
provided allocation order, and they can never return hints outside the
order. This is already documented in TargetRegisterInfo.h.

<rdar://problem/13240556>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175540 91177308-0d34-0410-b5e6-96231b3b80d8