Bug 1703469: Move getter/setter helpers inside IRGenerator r=jandem

This patch only moves code around, and should have no semantic changes.

Moving EmitGuardGetterSetterSlot inside the IR generator lets us access `isFirstStub_` in the same way as emitCalleeGuard, which will be used in the next patch.

Differential Revision: https://phabricator.services.mozilla.com/D251755

Author: iainireland