Bug in vector.lisp

[David.McDonald@WB1.CS.CMU.EDU]

There is a bug in loopup-pv-2 in vector.lisp.  The then part of the if:
	(aref cache (%+ offset 4))
should be changed to:
	(aref cache (%+ offset 3))

Otherwise, if there are two cache entries adjacent to one another and
the first one is accessed, the result will be the isl of the second
one and not the correct permutation vector for the first one.
		- David.