beam_ssa_opt: Add a pass for coalescing phi nodes

Nested cases can led to code such as this:

     10:
       _1 = phi {literal value1, label 8}, {Var, label 9}
      br 11

    11:
      _2 = phi {_1, label 10}, {literal false, label 3}

The phi nodes can be coalesced like this:

    11:
      _2 = phi {literal value1, label 8}, {Var, label 9},
               {literal false, label 3}

Coalescing can help other optimizations, and can in some cases reduce
register shuffling (if the phi variables for two phi nodes happens to
be allocated to different registers).

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