Add transforms for SORT and STABLE-SORT.

Transform SORT and STABLE-SORT into more specialized functions when
possible,
Improve LOGAND-BIGNUM/C VOP on x86-64.

Avoid putting unboxed constants into the code when possible.
Speed up iterative regalloc a little.

Avoid consing an intermediate list in vertex-color-possible-p.
Reduce consing in iterative regalloc.

The COLOR slot of VERTEX is defined as (cons color sc), but SC is
already accessible through the TN slot, as (tn-sc (vertex-tn vertex)).
Store only the color number in the COLOR slot, reducing consing.