[NaryReassociate] speeds up candidate searching

Summary:
This fixes a left-over efficiency issue in D8950.

As Andrew and Daniel suggested, we can store the candidates in a stack
and pop the top element when it does not dominate the current
instruction. This reduces the worst-case time complexity to O(n).

Test Plan: a new test in nary-add.ll that exercises this optimization.

Reviewers: broune, dberlin, meheff, atrick

Reviewed By: atrick

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D9055

llvm-svn: 235129