Eliminate the ability to add operands to an instruction, used in a narrow case

for SSA values in terminators, but easily worked around.  At the same time,
move the StmtOperand list in a OperationStmt to the end of its trailing
objects list so we can *reduce* the number of operands, without affecting
offsets to the other stuff in the allocation.

This is important because we want OperationStmts to be consequtive, including
their operands - we don't want to use an std::vector of operands like
Instructions have.

This is patch 4/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 226865727