This slipped through the regalloc2 operand code update in #4811: the
CvtFloatToUintSeq pseudo-instruction actually clobbers its source. It
was marked as a "mod" operand in the original and I mistakenly
converted it to a "use" as I had not seen the actual clobber. The
instruction now takes an extra temp and makes a copy of `src` in the
appropriate place.
Fixes#4840.