First part of refactoring ARM addrmode2 (load/store) instructions to be more

First part of refactoring ARM addrmode2 (load/store) instructions to be more
explicit about the operands. Split out the different variants into separate
instructions. This gives us the ability to, among other things, assign
different scheduling itineraries to the variants. rdar://8477752.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117409 91177308-0d34-0410-b5e6-96231b3b80d8