Revert "FIR checkers: report SMARTCAST_IMPOSSIBLE"

This reverts commit 84334b08

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/unstableSmartcastWhenOpenGetterWithOverloading.fir.kt

@@ -0,0 +1,15 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+interface Ctx
+class CtxImpl : Ctx {
+    fun doJob(a: Int) {}
+    fun doJob(s: String) {}
+}
+
+open class Test(open val ctx: Ctx) {
+    fun test() {
+        when (ctx) {
+            is CtxImpl -> ctx.<!UNRESOLVED_REFERENCE!>doJob<!>(2)
+        }
+    }
+}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/unstableSmartcastWhenOpenGetterWithOverloading.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 interface Ctx

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/unstableSmartcastWithOverloadedExtensions.fir.kt

@@ -0,0 +1,14 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class A
+class B
+
+fun A.foo(a: A) {}
+fun A.foo(b: B) {}
+var a: A? = null
+
+fun smartCastInterference(b: B) {
+    if (a != null) {
+        a<!UNSAFE_CALL!>.<!>foo(b)
+    }
+}

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/unstableSmartcastWithOverloadedExtensions.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 
 class A