`csimp` assumes constructors and `casesOn` applications match. That
is, given `I.casesOn x ...`, then if `x` is an constructor, then it is
a constructor of the inductive datatype `I`.
The transformation `erase_irrelevant` was violating this property when
it mixes `Decidable` and `Bool`. We fix this issue by mapping
`Decidable.casesOn`, `Decidable.isTrue` and `Decidable.isFalse` to
`Bool.casesOn`, `Bool.true` and `Bool.false` respectively.
