[SeparateConstOffsetFromGEP] garbage-collect intermediate instructions

Summary: so that we needn't run DCE after this pass.

Test Plan: removed -dce from the commandline in split-gep.ll and split-gep-and-gvn.ll

Reviewers: meheff

Subscribers: llvm-commits, HaoLiu, hfinkel, jholewinski

Differential Revision: http://reviews.llvm.org/D9096

llvm-svn: 235409