[mlir] Add folding of memref_cast inside another memref_cast

There exists a generic folding facility that folds the operand of a memref_cast
into users of memref_cast that support this. However, it was not used for the
memref_cast itself. Fix it to enable elimination of memref_cast chains such as

  %1 = memref_cast %0 : A to B
  %2 = memref_cast %1 : B to A

that is achieved by combining the folding with the existing "A to A" cast
elimination.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D90910