[mlir] Delete dup code and use unified methods.

[mlir] Delete dup code and use unified methods.

The foldMemRefCast method is defined in memref namespace; the
foldTensorCast method is defined in tensor namespace. This revision
deletes the dup code and use the unified methods.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D136379