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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159332
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