Fix frame pointer handling:

Reserve R0 in store/load from stack slot for building >32k offsets from SP
or FP.  This also requires we use R11 rather than R0 for holding the LR
value we want to save or restore.  Also, tell the register allocator not
to use R31 (our FP) in functions that have a frame pointer.  These changes
fix Burg.

llvm-svn: 15807