`SimplifyArm` and such are currently in `-Zunsound-mir-opts` and thus weren't running by default, so having something like them for the new desugar shouldn't be necessary for switching.
This optimization depends on inlining for the identity
conversions introduced by the lowering of the `?`.
To take advantage of `SimplifyArmIdentity`, `-Z mir-opt-level=2`
is required because that triggers the inlining MIR optimization.