X86: peephole optimization to remove cmp instruction

This patch will optimize the following:
  sub r1, r3
  cmp r3, r1 or cmp r1, r3
  bge L1
TO
  sub r1, r3
  bge L1 or ble L1

If the branch instruction can use flag from "sub", then we can eliminate
the "cmp" instruction.

llvm-svn: 157831