ARM: consolidate frame pointer register knowledge

Use the ARMBaseRegisterInfo to query the frame register.  The base register info
is aware of the frame register that is used for the frame pointer.  Use that to
determine the frame register rather than duplicating the knowledge.  Although,
the code path is slightly different in that it may return SP, that can only
occur if the frame pointer has been omitted in the machine function, which is
supposed to contain the desired value in that case.