[mlir] [transform] Error for duplicated processor mapping

In a nested loop nest, it is not feasible to map different loops to the same processing unit; for an example, check the code below. This modification includes a check in this circumstance.

```
scf.foreach_thread (%i, %j) in (%c32, %c32) {...}
{ mapping = [#gpu.thread<x>, #gpu.thread<x>] }

```

Note: It also deletes a test because it is not possible to reproduce this error.

Depends on D138020

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D138032