Revert "FIR checkers: report SMARTCAST_IMPOSSIBLE"

This reverts commit 84334b08

compiler/testData/diagnostics/tests/smartCasts/intersectionScope/unstableSmartCast.fir.kt

@@ -0,0 +1,37 @@
+// !CHECK_TYPE
+
+interface A {
+    fun foo(): CharSequence?
+    fun baz(x: Any) {}
+}
+
+interface B {
+    fun foo(): String
+    fun baz(x: Int): String =""
+    fun baz(x: Int, y: Int) {}
+
+    fun foobar(): CharSequence?
+}
+
+interface C {
+    fun foo(): String
+    fun baz(x: Int): String =""
+    fun baz(x: Int, y: Int) {}
+
+    fun foobar(): String
+}
+
+var x: A = null!!
+
+fun test() {
+    x.foo().checkType { _<CharSequence?>() }
+
+    if (x is B && x is C) {
+        x.foo().checkType { _<CharSequence?>() }
+        x.baz("")
+        x.baz(1).checkType { _<Unit>() }
+        x.baz(1, <!TOO_MANY_ARGUMENTS!>2<!>)
+
+        x.<!UNRESOLVED_REFERENCE!>foobar<!>().checkType { <!INAPPLICABLE_CANDIDATE!>_<!><String>() }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/intersectionScope/unstableSmartCast.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !CHECK_TYPE
 
 interface A {