Preserve types when swapping constant arguments and commute LOGTEST

 * Add a transform to ensure any constant argument to LOGTEST is in
   second position.

 * Commutative-arg-swap used to often cause suboptimal code: subsequent
   transforms fire before constraint propagation has tightened types
   back to their original value.  Hack with TRULY-THE for now. A more
   general fix (e.g. by declaring the type of arguments in spliced-in
   lambda expressions) would be even better.
Specialised VOPs for EQ of fixnum values on x86oids

 Steal fixnum EQL VOPs to implement EQ, like we already do for
 characters, words and signed-words: otherwise they're converted
 as signed-word EQ.
Improved SIMD-PACK manipulation VOPs on x86-64

 * Tighten naive (and, in one case, wrong-looking) lifetime
   specifications to enable more register coalescing.

 * Exploit the coalescing by replacing explicit move instructions
   with MOVE, and specify the SC to get MOVAPS for float data.

 * Microoptimise away some PSRLDQ by 0.