[K/N] Fixed a corner case in casts optimization pass

The alogrithm handles differently two types of variables - T? and Boolean.
The first type is used in expressions like if (var == null) { .. } or if (var != null) { .. },
while the other is used in just if (variable) { .. } or if (!variable) { .. }. But in one case,
a variable might change its type - when a Boolean? variable gets aliased to a Boolean variable.
This wasn't expected and there was an assertion for such a case (a variable can't change its kind).
This commit fixes this by supporting this peculiar corner case.

 #KT-81257 Fixed

Author: homuroll

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/optimizations/CastsOptimization.kt

@@ -642,7 +642,13 @@ internal class CastsOptimization(val context: Context) : BodyLoweringPass {
                                     )
                                 }
                                 is VariableValue.NullablePredicate -> variableValue.predicate
-                                is VariableValue.BooleanPredicate -> error("Unexpected boolean predicate for ${variable.render()}")
+                                is VariableValue.BooleanPredicate -> {
+                                    // Happens when a bool? variable aliases to a bool variable.
+                                    NullablePredicate(
+                                            ifNull = Predicate.False, // Never happens.
+                                            ifNotNull = Predicate.Empty
+                                    )
+                                }
                             }
 
             fun buildBooleanPredicate(variable: IrValueDeclaration): BooleanPredicate =