SDAG: Make SelectCodeCommon return void

SDAG: Make SelectCodeCommon return void

This means SelectCode unconditionally returns nullptr now. I'll follow
up with a change to make that return void as well, but it seems best
to keep that one very mechanical.

This is part of the work to have Select return void instead of an
SDNode *, which is in turn part of llvm.org/pr26808.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@269136 91177308-0d34-0410-b5e6-96231b3b80d8