target/riscv: Add DisasExtend to gen_arith*

target/riscv: Add DisasExtend to gen_arith*

Most arithmetic does not require extending the inputs.
Exceptions include division, comparison and minmax.

Begin using ctx->w, which allows elimination of gen_addw,
gen_subw, gen_mulw.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-7-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>