[Local] Clean up EmitGEPOffset

Handle the emission of the add in a single place, instead of three
different ones.

Don't emit an unnecessary add with zero to start with. It will get
dropped by InstCombine, but we may as well not create it in the
first place. This also means that InstCombine does not need to
specially handle this extra add.

This is conceptually NFC, but can affect worklist order etc.