Support declaring the same predecessor more than once in SSABulder

Extracted from https://github.com/0xPolygonMiden/miden-ir/pull/22#discussion_r1355858488

>A solution to this particular problem though is to split the edge in the predecessor, introducing new blocks along any duplicate edges which branch to the same successor, and which act as a passthrough for the "true" successor block. This gets you the desired behavior without the complexity of supporting the same block on more than one path in multi-way branching instructions.