[NaryReassoc] reassociate GEP for CSE

Summary:
x = &a[i];
y = &a[i + j];

=>

y = x + j;

along with some refactoring work such as extracting method
findClosestMatchingDominator.

Depends on D9786 which provides the ScalarEvolution::getGEPExpr interface.

Test Plan: nary-gep.ll

Reviewers: meheff, broune

Reviewed By: broune

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D9802

llvm-svn: 237971