Add a ARM specific pre-allocation pass that re-schedule loads / stores from

Add a ARM specific pre-allocation pass that re-schedule loads / stores from
consecutive addresses togther. This makes it easier for the post-allocation pass
to form ldm / stm.

This is step 1. We are still missing a lot of ldm / stm opportunities because
of register allocation are not done in the desired order. More enhancements
coming.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73291 91177308-0d34-0410-b5e6-96231b3b80d8