[Thumb-1] Select post-increment load and store where possible

Thumb-1 doesn't have post-inc or pre-inc load or store instructions. However the LDM/STM instructions with writeback can function as post-inc load/store:

  ldm r0!, {r1}  @ load from r0 into r1 and increment r0 by 4

Obviously, this only works if the post increment is 4.

llvm-svn: 275540