[mlir] Optimize the allocation of resizable operand lists

This revision optimizes resizable operand lists by allocating them in the same location as the trailing operands. This has numerous benefits:
* If the operation has at least one operand at construction time, there is 0 additional memory overhead to the resizable storage.
* There is less pointer arithmetic necessary as the resizable storage is now only used when the operands are dynamically allocated.

Differential Revision: https://reviews.llvm.org/D78854