[mlir][tosa] Add check if the operand of the operations is constant.

[mlir][tosa] Add check if the operand of the operations is constant.

Some uses of TOSA rely on the constant operands of particular operations,
e.g. paddings and pad_const in pad op. Add a verification pattern in the
validation pass, and this is optionally enabled.

Change-Id: I1628c0840a27ab06ef91150eee56ad4f5ac9543d

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D145412