The register list in vscclrm is unusual in three ways:
* The encoded size can be zero, meaning the list contains only vpr.
* Double-precision registers past d15 are permitted even when the
subtarget doesn't have them, they are instead ignored when the
instruction executes.
* The single-precision variant allows double-precision registers d16
onwards, which are encoded as a pair of single-precision registers.
Fixing this also incidentally changes a vlldm/vlstm error message: when
the first register is in the range d16-d31 we now get the "operand must
be exactly..." error instead of "register expected".
ad45eb4a9c