[BOLT] Make instruction size a first-class annotation (#72167)

When NOP instructions are used to reserve space in the code, e.g. for
patching, it becomes critical to preserve their original size while
emitting the code. On x86, we rely on "Size" annotation for NOP
instructions size, as the original instruction size is lost in the
disassembly/assembly process.

This change makes instruction size a first-class annotation and is
affectively NFCI. A follow-up diff will use the annotation for code
emission.