[X86] LowerRotate - enable vXi32 splat handling

Pull out the "rotl(x,y) --> (unpack(x,x) << zext(splat(y % bw))) >> bw" special case from vXi8 lowering so we can reuse it for vXi32 types as well.

There's still some regressions with vXi16 to handle before this becomes entirely general.

It also allows us to remove the now unnecessary hack for handling amount-modulo before splatting.