This change was introduced in 9a453338 and did not include specific tests. It was likely introduced to cope with the old restrictions regarding rewrite substitution of variables with different static broadcastable shape information, which was alleviated in #711

The behavior was already accounted by filter_variable which is called directly on as a fallback by the optimizer routines

Adds condition in convert_variable_test which would fail before this change

This rewrite predated partial shape specification in SpecifyShape, and as such ignored possible shape refinement over consecutive SpecifyShapes. It now merges information across consecutive SpecifyShape. Unlike the original, preference is given to the outer SpecifyShape, similar to what local_rebroadcast_lift does.