[LICM] Allow load-only scalar promotion in the presence of aliasing loads

During scalar promotion, if there are additional potentially-aliasing
loads outside the promoted set, we can still perform a load-only
promotion. As the stores are retained, any potentially-aliasing
loads will still read the correct value.

This increases the number of load promotions in llvm-test-suite by
a factor of two:

                                |  Old |  New
    licm.NumPromotionCandidates | 4448 | 6038
    licm.NumLoadPromoted        |  479 | 1069
    licm.NumLoadStorePromoted   | 1459 | 1459

Unfortunately, this does have some impact on compile-time:
http://llvm-compile-time-tracker.com/compare.php?from=57f7f0d6cf0706a88e1ecb74f3d3e8891cceabfa&to=72b811738148aab399966a0435f13b695da1c1c8&stat=instructions
In part this is because we now have less early bailouts from
promotion, but also due to second order effects (e.g. for one case
I looked at we spend more time in SLP now).

Differential Revision: https://reviews.llvm.org/D133192