[FIR] Add test cases to prove resolution of backlog tickets

^KT-41728 Fixed
^KT-46586 Fixed
^KT-49696 Fixed
^KT-51160 Fixed
^KT-53308 Fixed

compiler/testData/diagnostics/tests/smartCasts/varnotnull/capturedInlineModifiedBefore.kt

@@ -0,0 +1,15 @@
+// WITH_STDLIB
+// ISSUE: KT-46586
+
+inline fun run(block: () -> Unit) = block()
+
+fun test() {
+    var label: String? = null
+    run {
+        label = "zzz"
+    }
+    if (label == null) {
+        label = "zzz"
+    }
+    <!SMARTCAST_IMPOSSIBLE!>label<!>.isBlank()
+}