[mlir] allow repeated payload in structured.fuse_into_containing

Structured fusion proceeds by iteratively finding the next suitable
producer to be fused into the loop. Therefore, it shouldn't matter if
the same producer is listed multiple times (e.g., it is used as multiple
operands). Adjust the implementation of the transform op to support this
case.

Also fix the checking code in the interpreter to actually respect the
TransformOpInterface indication that repeated payload is allowed, it
seems to have been accidentally dropped in one of the refactorings.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D150561