KT-14581 Make FixStackAnalyzer tolerant to uninitialized values

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

@@ -0,0 +1,10 @@
+fun box(): String {
+    val ok: String? = "OK"
+    var res = ""
+
+    do {
+        res += ok ?: break
+    } while (false)
+
+    return res
+}

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

@@ -0,0 +1,6 @@
+fun box(): String {
+    var i = 0
+    do continue while (i++ < 3)
+
+    return "OK"
+}

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")