Inline Instruction's operands as TrailingObjects

For performance/memory saving purpose, having the Instruction holding a
std::vector for the operands isn't a really good tradeoff.  The only reason for
this was to support adding/removing easily BasicBlock arguments to Terminator.
Since this isn't the most common operation, we instead force a pre-allocated
list of operands on Instructions at creation time.
PiperOrigin-RevId: 226981227