Hm, this special handling for Combined->Image+Alpha is really a PITA...

So, the problem here was that the Image output was reused since it already exists, but was configured to use the Combined pass. However, I just checked how 2.78c handles this situation, and it turns out that when you just F3-save a rendered image and put it into a Image node there, it gives you two passes named "Combined" and "Alpha", not Image and Alpha.

Therefore, one potential fix is to rename the default "Image" output to "Combined" for the Image node (not the RenderLayer node), which 1. makes the socket name->pass name mapping consistent and 2. makes behaviour similar to 2.78c.
One difference is that with that fix (P481) and the file from this report, the Image pass is displayed as the last socket, not the first as in 2.78c. But, the output ordering isn't really well-defined anyways for the Image node with arbitrary multilayer inputs, so I consider that acceptable.

@Sergey Sharybin (sergey), sounds good for you?

The reason why saving multi-layer layer with F3 gives different name is because Blender uses passes names for EXR passes. But you can call passes arbitrary in EXR, could also be "mammonth's banana". So not sure how this indicates a problem?

As for rename: i don't see how "Combined" socket name makes any sense for non-multilayer images (such as PNG i.e.). This is misleading and is not consistent with other node types (Movie Clip node in Compositor, Image node in texture node. Cycles shading is different here -- it calls everything a color). For multi-layer images we should be using names from EXR.