PR9634: Don't unconditionally tell the AliasSetTracker that the PreheaderLoad

PR9634: Don't unconditionally tell the AliasSetTracker that the PreheaderLoad
is equivalent to any other relevant value; it isn't true in general.
If it is equivalent, the LoopPromoter will tell the AST the equivalence.
Also, delete the PreheaderLoad if it is unused.

Chris, since you were the last one to make major changes here, can you check
that this is sane?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129049 91177308-0d34-0410-b5e6-96231b3b80d8