Remove uses of get_slice_int() in vector code?

[Timmmm]

In the vector code there are some uses of get_slice_int() for multiplies:

        MVV_VMUL     => get_slice_int(SEW, signed(vs2_val[i]) * signed(vs1_val[i]), 0),
        MVV_VMULH    => get_slice_int(SEW, signed(vs2_val[i]) * signed(vs1_val[i]), SEW),
        MVV_VMULHU   => get_slice_int(SEW, unsigned(vs2_val[i]) * unsigned(vs1_val[i]), SEW),
        MVV_VMULHSU  => get_slice_int(SEW, signed(vs2_val[i]) * unsigned(vs1_val[i]), SEW),
...


In the non-vector multiply we do it differently with `to_bits_truncate()`, like this:

let rs1_int = if mul_op.signed_rs1 then signed(rs1_bits) else unsigned(rs1_bits);
let rs2_int = if mul_op.signed_rs2 then signed(rs2_bits) else unsigned(rs2_bits);

let result_wide = to_bits_truncate(2 * xlen, rs1_int * rs2_int);
X(rd) = if mul_op.high
then result_wide[(2 * xlen - 1) .. xlen]
else result_wide[(xlen - 1) .. 0];

We should do it the same in both places, and maybe extract it in a function.