[mlir][memref] Fix an invalid dim loop motion crash (#74204)

Fixes https://github.com/llvm/llvm-project/issues/73382.

This PR suggests to replace two assertions that were introduced in
https://github.com/llvm/llvm-project/commit/adabce41185910227ca276a1cfd22e76443dd238
(https://reviews.llvm.org/D135748). According to the enum definition of
`NotSpeculatable`, an op that invokes undefined behavior is
`NotSpeculatable`.

https://github.com/llvm/llvm-project/blob/0c06e8745f131d867c566f4d35a7a04e24b4a075/mlir/include/mlir/Interfaces/SideEffectInterfaces.h#L248-L258

and both `tensor.dim` and `memref.dim` state that "If the dimension
index is out of bounds, the behavior is undefined."

So therefore it seems to me that `DimOp::getSpeculatability()` should
return `NotSpeculatable` if the dimension index is out of bounds.

The added test is just a simplified version of
https://github.com/llvm/llvm-project/issues/73382.