dominance bug

upstream MLIR rejects this as a dominance violation. we accept it-- I believe incorrectly

```mlir
"func.func"() <{sym_name = "f", function_type = (i1, i1, i1) -> ()}> ({
^entry(%a : i1, %b : i1, %c : i1):
"cf.cond_br"(%a, %a) [^left, ^def] <{operandSegmentSizes = array}> : (i1, i1) -> ()
^left:
"cf.br"() [^join] : () -> ()
^def(%v : i1):
"cf.br"() [^use] : () -> ()
^mid(%unused : i1):
"cf.br"() [^tail] : () -> ()
^use:
"cf.br"(%v) [^mid] : (i1) -> ()
^join:
"cf.cond_br"(%b, %a) [^mid, ^left] <{operandSegmentSizes = array}> : (i1, i1) -> ()
^tail:
"cf.cond_br"(%c) [^join, ^use] <{operandSegmentSizes = array}> : (i1) -> ()
}) : () -> ()
```