Precompute SCEV pointer analysis prior to instruction fusion in BBVectorize.

When both a load/store and its address computation are being vectorized, it can
happen that the address-computation vectorization destroys SCEV's ability
to analyize the relative pointer offsets. As a result (like with the aliasing
analysis info), we need to precompute the necessary information prior to
instruction fusing.

This was found during stress testing (running through the test suite with a very
low required chain length); unfortunately, I don't have a small test case.

llvm-svn: 159332