1.0.38.6: Clear higher order bits for SSE operations that don't

1.0.38.6: Clear higher order bits for SSE operations that don't

 * SQRTSD, CVT{SS,SD}2{SS,SD} and CVTSI2{SS,SD} leave the high-order
   bits of the result as-is. These are the only (hopefully) operations
   we use that have a single input, so we must explicitly clear out
   the destination register.

 * Probably a performance fix too, as it breaks dependency chains on
   the destination register.

 * The bug was relatively hard to observe. Raymond Toy reported seeing
   and fixing something similar on maxima/CMUCL. Brittle test case
   added.