For ARM stack frames that utilize variable sized objects and have either

large local stack areas or require dynamic stack realignment, allocate a
base register via which to access the local frame. This allows efficient
access to frame indices not accessible via the FP (either due to being out
of range or due to dynamic realignment) or the SP (due to variable sized
object allocation). In particular, this greatly improves efficiency of access
to spill slots in Thumb functions which contain VLAs.

rdar://7352504
rdar://8374540
rdar://8355680

llvm-svn: 112883