[mlir][tosa] Fix segmentation fault in case of folding unranked tensor

Trying to fold the unranked tensor for "tosa.equal" crashes due to null reference.
We need to check the dynamic cast result beforehand. This is reported in
https://github.com/llvm/llvm-project/issues/60192.

Reviewed By: rsuderman

Differential Revision: https://reviews.llvm.org/D143034