[mlir] Move the Operation OperandStorage to the first trailing object

The main benefits of this change are faster access to operands
(no need to compute the offset, as it is now right after the
operation), simpler code(no need to manage a lot of the "is the
operand storage trailing" logic we had to before). The major
downside to this though, is that operand holding operations now
grow in size by 1 word (as no matter how we do this change, there
will need to be some additional book keeping).

Differential Revision: https://reviews.llvm.org/D111695