fixed bug in constraint system

For parameter type T constraint T? <: Int? should NOT transform to T <: Int, it should be T <: Int?
equality constraint T? = Int? should transform to T <: Int? && T >: Int

compiler/testData/diagnostics/tests/inference/nullableUpperBound.kt

@@ -0,0 +1,9 @@
+package o
+
+fun foo(): String? {
+    return accept(JV<String?, Unit?>())
+}
+
+fun <R, D> accept(<!UNUSED_PARAMETER!>v<!>: JV<R, D>): R<!BASE_WITH_NULLABLE_UPPER_BOUND!>?<!> = null
+
+open class JV<R, D>()