[NaryReassociate] Run EarlyCSE after NaryReassociate

[NaryReassociate] Run EarlyCSE after NaryReassociate

Summary:
This patch made two improvements to NaryReassociate and the NVPTX pipeline

1. Run EarlyCSE/GVN after NaryReassociate to get rid of redundant common
expressions.

2. When adding an instruction to SeenExprs, maps both the SCEV before and after
reassociation to that instruction.

Test Plan: updated @reassociate_gep_nsw in nary-gep.ll

Reviewers: meheff, broune

Reviewed By: broune

Subscribers: dberlin, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D9947

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238396 91177308-0d34-0410-b5e6-96231b3b80d8