Revert "r251451 - [AliasSetTracker] Use mod/ref information for UnknownInstr"

It looks like this broke the stage 2 builder:
  http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto/6989/

Original commit message:

AliasSetTracker does not need to convert the access mode to ModRefAccess if the
new visited UnknownInst has only 'REF' modrefinfo to existing pointers in the
sets.

Patch by Andrew Zhogin!

llvm-svn: 251562