target/riscv/vector_helper.c: optimize loops in ldst helpers

target/riscv/vector_helper.c: optimize loops in ldst helpers

Change the for loops in ldst helpers to do a single increment in the
counter, and assign it env->vstart, to avoid re-reading from vstart
every time.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240314175704.478276-11-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>