CastOptimizer: don't assume dynamic casts from ObjectiveC classes to unrelated classes will fail

[eeckstein]

In case of ObjectiveC classes, the runtime type can differ from its declared type. Therefore a cast between (compile-time) unrelated classes may succeed at runtime.

rdar://149810124

[mikeash]

Approving with sadness.

[eeckstein]

@swift-ci apple silicon benchmark

[tbkka]

For anyone curious about this change, consider this code:

class Foo {}
class Bar {}

let foo: Foo = ...

if foo is Foo { print("Foo") }
else if foo is Bar { print("Bar") }

"Obviously", the casts here can be optimized away and we're left with just print("Foo"). But in fact, if Foo and Bar are Obj-C class types, then we can't optimize this, because foo might really be a Bar -- Objective-C lets programmers get away with things like that. We're still going to optimize this for Swift types, though, since this situation occurs surprisingly often in inlined code.

[tbkka]

@swift-ci Please test