[mlir][memref] Lower copy of memrefs with outer size-1 dims to intrinsic memcpy.

With this change, more `memref.copy` will be lowered to the efficient `memcpy`. For example,

```
memref.copy %subview, %alloc : memref<1x576xf32, strided<[704, 1]>> to memref<1x576xf32>
```

Differential Revision: https://reviews.llvm.org/D150448