two changes: 1) make AliasSet hold the list of call sites with an

two changes: 1) make AliasSet hold the list of call sites with an
assertingvh so we get a violent explosion if the pointer dangles.

2) Fix AliasSetTracker::deleteValue to remove call sites with
   by-pointer comparisons instead of by-alias queries.  Using
   findAliasSetForCallSite can cause alias sets to get merged
   when they shouldn't, and can also miss alias sets when the
   call is readonly.

#2 fixes PR6889, which only repros with a .c file :(

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112452 91177308-0d34-0410-b5e6-96231b3b80d8