Remove the need to cache the subtarget in the ARM TargetRegisterInfo

Remove the need to cache the subtarget in the ARM TargetRegisterInfo
classes. Replace the frame pointer initialization with a static function
that'll look it up via the subtarget on the MachineFunction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232010 91177308-0d34-0410-b5e6-96231b3b80d8