DAGCombine's logic for forming pre- and post- indexed loads / stores were being

overly conservative. It was concerned about cases where it would prohibit
folding simple [r, c] addressing modes. e.g.
  ldr r0, [r2]
  ldr r1, [r2, #4]
=>
  ldr r0, [r2], #4
  ldr r1, [r2]
Change the logic to look for such cases which allows it to form indexed memory
ops more aggressively.

rdar://10674430

llvm-svn: 148086