perform DSE through launder.invariant.group

perform DSE through launder.invariant.group

Summary:
Alias Analysis knows that llvm.launder.invariant.group
returns pointer that mustalias argument, but this information
wasn't used, therefor we didn't DSE through launder.invariant.group

Reviewers: chandlerc, dberlin, bogner, hfinkel, efriedma

Reviewed By: dberlin

Subscribers: amharc, llvm-commits, nlewycky, rsmith

Differential Revision: https://reviews.llvm.org/D31581

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331449 91177308-0d34-0410-b5e6-96231b3b80d8