[mlir][SCF] Add `SingleBlock` op trait to "scf.while"

This trait is needed so that unstructured control flow is not inlined into "scf.while" ops.

Note: The two regions of "scf.while" are already defined as `SizedRegion<1>`. `SingleBlock` can be queried from C++, `SizedRegion<n>` not.

Fixes #64976.

Differential Revision: https://reviews.llvm.org/D159199