Add disassembly option to be forgiving for invalid binary.

Currently the spirv-asm format allows invalid 32bit words to be recorded (https://github.com/KhronosGroup/SPIRV-Tools/blob/master/syntax.md#arbitrary-integers) the disassembler should provide an option to emit this format if the given words are unknown.

If the invalid words are the opcode/word count there isn't much we can do, but for other words in the binary we should be able to output the hex variant as a fallback.

Start with syntax.md and its section on arbitrary integers to understand the permitted hex fallback format. Then locate the disassembler path that handles unknown words and verify the behavior for invalid operands versus invalid opcodes or word counts; done means supported invalid words emit hex while unrecoverable instruction structure remains unchanged.

Written by the indexing model from the issue text.