target/riscv: rvv-1.0: integer comparison instructions

* Sign-extend vmselu.vi and vmsgtu.vi immediate values.
* Remove "set tail elements to zeros" as tail elements can be unchanged
  for either VTA to have undisturbed or agnostic setting.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211210075704.23951-48-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>