[NaryReassociate] run NaryReassociate iteratively

[NaryReassociate] run NaryReassociate iteratively

Summary:
An alternative is to use a worklist approach. However, that approach
would break the traversing order so that we couldn't lookup SeenExprs
efficiently. I don't see a clear winner here, so I picked the easier approach.

Along with two minor improvements:
1. preserves ScalarEvolution by forgetting instructions replaced
2. removes dead code locally avoiding the need of running DCE afterwards

Test Plan: add to slsr-add.ll a test that requires multiple iterations

Reviewers: broune, dberlin, atrick, meheff

Reviewed By: atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9058

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235151 91177308-0d34-0410-b5e6-96231b3b80d8