[ARM] Shrink post-indexed LDR and STR to LDM/STM

A Thumb-2 post-indexed LDR instruction such as:

  ldr.w r0, [r1], #4

Can be rewritten as:

  ldm.n r1!, {r0}

LDMs can be more expensive than LDRs on some cores, so this has been enabled only in minsize mode.

llvm-svn: 272002