Clean up listify-rest-args VOP on x86-64.

It's no longer using loop instructions, remove STD and CLD.
Micro-optimize copy-more-arg on x86-64.

Instead of copying RCX into RBX, then modifying RCX and later
restoring RCX from RBX, modify RBX instead.
Clean up and micro-optimize list checking in some x86-64 VOPs.

In length/list and values-list, instead of manually checking for LIST,
call %test-lowtag, which produces more compact code.