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.

The issue references a discussion from a pull request in the miden-ir repository. Start by examining the SSABuilder code to understand how predecessors are currently handled. Look at the linked PR comment for context on splitting edges and introducing passthrough blocks. The goal is to modify the builder to allow duplicate predecessor declarations without breaking existing SSA construction.

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