This isn't supported directly so we rotate the vector by the desired number of
elements, insert to element zero, then rotate back.
The i64 case generates rather poor code on MIPS32. There is an obvious
optimisation to be made in future (do both insert.w's inside a shared
rotate/unrotate sequence) but for now it's sufficient to select valid code
instead of aborting.
Depends on D3536
This might be a dumb question but why MSA128WRegClass ? Would MSA128DRegClass also be OK ?