[FIR] Fold flexible types after substitution if possible

compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSam.txt

@@ -8,7 +8,7 @@ FILE: main.kt
             ^ R|<local>/x|.R|kotlin/Any.toString|()
         }
         )
-        R|/MyFunction|<R|kotlin/Nothing|, R|kotlin/Any?|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Nothing, kotlin/Nothing?>!|): R|ft<kotlin/Any?, kotlin/Any?>?| {
+        R|/MyFunction|<R|kotlin/Nothing|, R|kotlin/Any?|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Nothing, kotlin/Nothing?>!|): R|kotlin/Any?| {
             ^ String()
         }
         )

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/substitution/Substitutors.kt

@@ -50,7 +50,12 @@ abstract class AbstractConeSubstitutor : ConeSubstitutor() {
             is ConeClassErrorType -> return null
             is ConeClassLikeType -> this.substituteArguments()
             is ConeLookupTagBasedType -> return null
-            is ConeFlexibleType -> this.substituteBounds()
+            is ConeFlexibleType -> this.substituteBounds()?.let {
+                // TODO: may be (?) it's worth adding regular type comparison via AbstractTypeChecker
+                // However, the simplified check here should be enough for typical flexible types
+                if (it.lowerBound == it.upperBound) it.lowerBound
+                else it
+            }
             is ConeCapturedType -> return null
             is ConeDefinitelyNotNullType -> this.substituteOriginal()
             is ConeIntersectionType -> this.substituteIntersectedTypes()

compiler/tests-spec/testData/diagnostics/linked/expressions/conditional-expression/p-6/neg/1.1.fir.kt

@@ -53,7 +53,7 @@ public class JavaClassCase3{
 // TESTCASE NUMBER: 3
 fun case3() {
     val x = JavaClassCase3.id(null) // Nothing!
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?..kotlin.Nothing??")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>x<!>
     val a = if (x) {
         "NOK"
     } else "NOK"