target/riscv: rvv-1.0: add VMA and VTA

target/riscv: rvv-1.0: add VMA and VTA

Introduce vma and vta fields in vtype register.

According to RVV 1.0 spec (section 3.3.3):

When a set is marked agnostic, the corresponding set of destination
elements in any vector or mask destination operand can either retain
the value they previously held, or are overwritten with 1s.

So, either vta/vma is set to undisturbed or agnostic, it's legal to
retain the inactive masked-off elements and tail elements' original
values unchanged. Therefore, besides declaring vta/vma fields in vtype
register, also remove all the tail elements clean functions in this
commit.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-15-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>