FIR: Refine inference constraints when type variable in flexible position

That issue might be fixed via changing
TypeVariableMarker.shouldBeFlexible at ConeConstraintSystemUtilContext
but this and some other tricks have been added because of incorrect
handling of constraints where type variable has a flexible bound

^KT-51168 Fixed

compiler/testData/diagnostics/tests/j+k/flexibleTypeVariablePosition.fir.kt

@@ -0,0 +1,20 @@
+// SKIP_TXT
+// FILE: JavaClass.java
+public class JavaClass {
+    public static <K> K simpleId(K k) { // fun <K> simpleId(k: K & Any..K?): K & Any..K? =
+        return k;
+    }
+}
+
+// FILE: main.kt
+
+fun takeN(n: Number?): Int = 1
+
+fun bar(n: Number?) {
+    fun takeN(n: Number): String = ""
+
+    // in K1, it was resolved to nullable takeN
+    // in K2, it would be resolved to not-nullable and may fail with NPE
+    takeN(JavaClass.simpleId(n)).<!UNRESOLVED_REFERENCE!>div<!>(1)
+    takeN(JavaClass.simpleId(n)).length
+}

compiler/testData/diagnostics/tests/j+k/flexibleTypeVariablePosition.kt

@@ -0,0 +1,20 @@
+// SKIP_TXT
+// FILE: JavaClass.java
+public class JavaClass {
+    public static <K> K simpleId(K k) { // fun <K> simpleId(k: K & Any..K?): K & Any..K? =
+        return k;
+    }
+}
+
+// FILE: main.kt
+
+fun takeN(n: Number?): Int = 1
+
+fun bar(n: Number?) {
+    fun takeN(n: Number): String = ""
+
+    // in K1, it was resolved to nullable takeN
+    // in K2, it would be resolved to not-nullable and may fail with NPE
+    takeN(JavaClass.simpleId(n)).div(1)
+    takeN(JavaClass.simpleId(n)).<!UNRESOLVED_REFERENCE!>length<!>
+}

compiler/testData/diagnostics/tests/j+k/types/upperBoundsCheckAgainstSelfTypeInJava.kt

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+// FILE: XBreakpointProperties.java
+public abstract class XBreakpointProperties<T> {}
+// FILE: XBreakpoint.java
+public interface XBreakpoint<P extends XBreakpointProperties> {}
+// FILE: XBreakpointType.java
+public abstract class XBreakpointType<B extends XBreakpoint<P>, P extends XBreakpointProperties> {}
+
+// FILE: main.kt
+fun foo(x: XBreakpointType<XBreakpoint<*>, *>) {}