GENTAG/ADDTAG generating zero-tags

[cmuellner]

Zero-tags have a special meaning as they are the default tags:

• an unset memory tag is expected to be zero
• an unset pointer tag is zero

This raises the question of whether GENTAG/ADDTAG should generate a zero-tag as this leads to tagged-memory being potentially equally tagged to non-tagged-memory.

I would prefer to have tagged memory never be compatible with zero-tagged pointers.
This could be realised by assuring that GENTAG does not generate zero-tags and ADDTAG would skip zero-tags.