[mlir] [memref] Fix alignment bug in memref.copy lowering

memref.copy gets lowered to a function call sometimes, this function
is passed the element size of the memref in bytes as an argument.
The element size passed to the copyMemRef() function call can be
miscalculated if the LLVM IR uses aligned access to the memory.

This can be fixed by using llvm.getelementptr to calculate the element
size natively. This is also done in the other lowering path that lowers
to an intrinsic.

Fix https://github.com/llvm/llvm-project/issues/64072

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D156126