[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

llvm-svn: 235151