BBVectorize: Cache fixed-order pairs instead of recomputing pointer info.

Instead of recomputing relative pointer information just prior to fusing,
cache this information (which also needs to be computed during the
candidate-pair selection process). This cuts down on the total number of
SE queries made, and also is a necessary intermediate step on the road toward
including shuffle costs in the pair selection procedure.

No functionality change is intended.

llvm-svn: 167049