[NaryReassociate] allow candidate to have a different type

Summary:
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

llvm-svn: 256035