[Bug] Incorrect scf.for code is generated

[menghaining]

The following C code generates wrong mlir code

int foo(int num) {
  int sum = 0;
  int i = 0;
  for (; i < num; i += 5) {
    sum += i;
  }
  return sum + i;
}

func.func @_Z3fooi(%arg0: i32) -> i32 attributes {llvm.linkage = #llvm.linkage<external>} {
  %c0 = arith.constant 0 : index
  %c5 = arith.constant 5 : index
  %c0_i32 = arith.constant 0 : i32
  %0 = arith.index_cast %arg0 : i32 to index
  %1 = scf.for %arg1 = %c0 to %0 step %c5 iter_args(%arg2 = %c0_i32) -> (i32) {
    %3 = arith.index_cast %arg1 : index to i32
    %4 = arith.addi %arg2, %3 : i32
    scf.yield %4 : i32
  }
  %2 = arith.addi %1, %arg0 : i32
  return %2 : i32
}

This instruction %2 = arith.addi %1, %arg0 : i32 is wrong because num is not necessarily a multiple of 5