[BOLT] Modify MCPlus annotation internals. NFCI. (#70412)

When annotating MCInst instructions, attach extra annotation operands
directly to the annotated instruction, instead of attaching them to an
instruction pointed to by a special kInst operand.

With this change, it's no longer necessary to allocate MCInst and most
of the first-class annotations come with free memory as currently MCInst
is declared with:

    SmallVector<MCOperand, 10> Operands;

i.e. more operands than are normally being used.

We still create a kInst operand with a nullptr instruction value to
designate the beginning of annotation operands. However, this special
operand might not be needed if we can rely on MCInstrDesc::NumOperands.