[NI] Avoid constraints from expected type for effectively empty system

Expression will be checked against expected type later.

 Theoretically, this is not very good, but it aligns with the old
 inference, plus it helps avoiding multiple type mismatch diagnostics.

compiler/testData/diagnostics/tests/inference/coercionToUnit/coercionToUnitForIfAsLastExpressionInLambda.kt

@@ -12,7 +12,7 @@ fun foo(): String? {
     run {
         if (true) return@run
 
-        if (true) return <!TYPE_MISMATCH, TYPE_MISMATCH!>Obj()<!> // correct error, type check against return type of function "foo"
+        if (true) return <!TYPE_MISMATCH!>Obj()<!> // correct error, type check against return type of function "foo"
     }
 
     run {

compiler/testData/diagnostics/tests/inference/kt1293.kt

@@ -5,7 +5,7 @@ package kt1293
 
 fun main() {
     val intArray = arrayOfNulls<Int>(10)
-    val <!UNUSED_VARIABLE!>i<!> : Int = <!NI;TYPE_MISMATCH, TYPE_MISMATCH!>intArray[0]<!>
+    val <!UNUSED_VARIABLE!>i<!> : Int = <!TYPE_MISMATCH!>intArray[0]<!>
     requiresInt(<!TYPE_MISMATCH!>intArray[0]<!>)
 }
 

compiler/testData/diagnostics/tests/inference/regressions/kt2883.kt

@@ -29,5 +29,5 @@ fun testWithoutInference(col: MutableCollection<Int>) {
     
     doAction { col.add(2) }
     
-    val <!UNUSED_VARIABLE!>u<!>: Unit = <!TYPE_MISMATCH!>col.<!NI;TYPE_MISMATCH!>add(2)<!><!>
+    val <!UNUSED_VARIABLE!>u<!>: Unit = <!TYPE_MISMATCH!>col.add(2)<!>
 }

compiler/testData/diagnostics/tests/inference/upperBounds/useBoundsIfUnknownParameters.kt

@@ -8,5 +8,5 @@ class Client<T, X: Base<T>>(<!UNUSED_PARAMETER!>x<!>: X)
 
 fun test() {
     val c = Client(StringBase()) // Type inference fails here for T.
-    val <!UNUSED_VARIABLE!>i<!> : Int = <!NI;TYPE_MISMATCH, TYPE_MISMATCH!>c<!>
+    val <!UNUSED_VARIABLE!>i<!> : Int = <!TYPE_MISMATCH!>c<!>
 }