target/riscv: rvv-1.0: integer add-with-carry/subtract-with-borrow

target/riscv: rvv-1.0: integer add-with-carry/subtract-with-borrow

* Only do carry-in or borrow-in if is masked (vm=0).
* Remove clear function from helper functions as the tail elements
  are unchanged in RVV 1.0.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-44-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>