Remove the restriction that only registered terminator operations may...

Remove the restriction that only registered terminator operations may terminate a block and have block operands. This allows for any operation to hold block operands. It also introduces the notion that unregistered operations may terminate a block. As such, the 'isTerminator' api on Instruction has been split into 'isKnownTerminator' and 'isKnownNonTerminator'.

PiperOrigin-RevId: 233076831