AArch64: Combine add/sub with comparison against zero

For Java code block like "if (a + b > 0) {...}", currently add+cmp instructions are generated.
  add w10, w2, w1
  cmp w10, #0x0
  b.gt 0x0000ffff790d5274
Indeed, these two instructions can be combined into "cmn w2, w1" (alias of adds); sub+cmp can be combined into a single cmp (alias of subs) as well.