[FIR] Introduce unchecked not-null cast as internal operation for !!, ?:

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt

@@ -197,7 +197,10 @@ internal fun FirExpression.generateNotNullOrOther(
         )
         branches += FirWhenBranchImpl(
             session, other.psi, FirElseIfTrueCondition(session, psi),
-            FirSingleExpressionBlock(session, generateResolvedAccessExpression(session, psi, subjectVariable))
+            FirSingleExpressionBlock(
+                session,
+                FirUncheckedNotNullCastImpl(session, psi, generateResolvedAccessExpression(session, psi, subjectVariable))
+            )
         )
     }
 }

compiler/fir/psi2fir/testData/rawBuilder/expressions/nullability.txt

@@ -5,7 +5,7 @@ FILE: nullability.kt
                 Int(42)
             }
             else ->  {
-                R|<local>/<elvis>|
+                R|<local>/<elvis>|!
             }
         }
 
@@ -16,7 +16,7 @@ FILE: nullability.kt
                 throw KotlinNullPointerException#()
             }
             else ->  {
-                R|<local>/<bangbang>|
+                R|<local>/<bangbang>|!
             }
         }