target/riscv: rvv: Add tail agnostic for vector load / store instructions

target/riscv: rvv: Add tail agnostic for vector load / store instructions

Destination register of unit-stride mask load and store instructions are
always written with a tail-agnostic policy.

A vector segment load / store instruction may contain fractional lmul
with nf * lmul > 1. The rest of the elements in the last register should
be treated as tail elements.

Signed-off-by: eop Chen <eop.chen@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <165449614532.19704.7000832880482980398-6@git.sr.ht>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>