[Mosaic GPU] Fix bug in layout inference of WGMMA.
Previously, we chose the largest possible swizzle value in order to determine
the tile size along the minormost dimension of the WGMMA's SMEM operands.
While using the largest tiling possible is a reasonable thing to do, it is not
strictly necessary for correctness. As a result, it is possible for a user to
specify a smaller tiling, and still produce a valid program. In this case, our
strict assignment logic immediately made the system unsatisfiable, despite the
existence of a valid solution that would satisfy the user's constraints.
This change replaces the assignment with a new constraint `IsValidMmaTiling`.
This allows applying the user-specified constraint if it exists, and also
allows us to still conjure the largest possible tiling if none is specified.
A similar fix is necessary for `tcgen05_mma`, and will come in a separate
change.
PiperOrigin-RevId: 863368353
