[mlir][Linalg] Avoid changing the rank of the result in canonicalizations of subtensor.

Canonicalizations for subtensor operations defaulted to use the
rank-reduced version of the operation, but the cast inserted to get
back the original type would be illegal if the rank was actually
reduced. Instead make the canonicalization not reduce the rank of the
operation.

Differential Revision: https://reviews.llvm.org/D101258