finalize the semantics of those opcodes with potentially undefined behavior

[robertmuth]

• mod with signed ints
• div/mod with zero divisors
• shifts with shift amounts exceeding the bitwidth of the operand
• conversions that widen and change from signed to unsigned

see comments interleaved with opcode description.

[robertmuth]

suggestion from a reddit user

Don't forget to maintain the identity:

div a 0 -> 0
mod a 0 -> a
(it helps to think of "0" as "2bitwidth, but wrapped around")