[InstCombine, SCCP] Consolidate code used to remove instructions

InstCombine and SCCP both want to remove dead code in a very particular
way but using identical means to do so.  Share the code between the two.

No functionality change is intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258653 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed