KT-14581 Make FixStackAnalyzer tolerant to uninitialized values

compiler/testData/codegen/box/controlStructures/breakContinueInExpressions/kt14581.kt

@@ -0,0 +1,11 @@
+fun foo(x: String): String {
+    var y: String
+    do {
+        y = x
+    } while (y != x.bar(x))
+    return y
+}
+
+inline fun String.bar(other: String) = this
+
+fun box(): String = foo("OK")