Definedness improvements

[bbyalcinkaya]

• Mark unary int ops and int extension ops as total.
• Check definedness of br_table and add preserves-definedness

[jberthold]

FYI I am running a test with these changes (even though the calls are not protected against arguments being negative yet). My test includes another small change (marking the load instruction for signed result as preserving definedness). Maybe you could consider adding it to this PR, or I can open another one.