Shift args

[marcfedorow]

The following will simplify shifter logic (and also allow to merge SIMD and regular shifters easier):

1. In imm shifts (as kslli8) let chunk[25:20+len(imm)] bits be 0;
2. In reg shifts (as ksll8) if val(rs2) is not in shift range, result is implementation defined.
   KSLRA's input should be properly sign extended; no exceptions like sa = (sa == 8)? 7 : sa;.

This will be somehow similar to C/C++ standards where int64_t >> 64 is an UB.