Update
Apple has clarified that slicing occurs independent of enabling bitcode. I've observed this in practice as well where a non-bitcode enabled app will only be downloaded as the architecture appropriate for the target device.
Original
Bitcode. Archive your app for submission to the App Store in an intermediate representation, which is compiled into 64- or 32-bit executables for the target devices when delivered.
Slicing. Artwork incorporated into the Asset Catalog and tagged for a platform allows the App Store to deliver only what is needed for installation.
The way I read this, if you support bitcode, downloaders of your app will only get the compiled architecture needed for their own device.