ARM: Allocation hints must make sure to be in the alloc order.

When creating an allocation hint for a register pair, make sure the hint
for the physical register reference is still in the allocation order.

rdar://13240556

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175541 91177308-0d34-0410-b5e6-96231b3b80d8