[NaryReassociate] allow candidate to have a different type
If Candiadte may have a different type from GEP, we should bitcast or
pointer cast it to GEP's type so that the later RAUW doesn't complain.
Added a test in nary-gep.ll
Reviewers: tra, meheff
Subscribers: mcrosier, llvm-commits, jholewinski
Differential Revision: http://reviews.llvm.org/D15618