Optimize FILL-POINTER function.

Declare that the FILL-POINTER-ERROR function called by the inline
expansion of FILL-POINTER doesn't return.
Make the second argument of FILL-POINTER-ERROR optional, so that
fill-pointer doesn't have to pass a NIL.
Micro-optimize CONSP and SYMBOLP VOPs on x86.

Save one byte by using the CMP instruction on the EAX register.