[FIR] Remove the fast path if as insignificant

This if is a bit inconsistent with the
LUB checking approach, so if it doesn't
improve performance it's better to remove it.

compiler/fir/analysis-tests/testData/resolveWithStdlib/intellij/IntersectionWithJavaString.kt

@@ -5,5 +5,5 @@
 
 fun collapse(path: String) {
     val result = (path as <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.String<!>).replace("123", "456")
-    if (<!EQUALITY_NOT_APPLICABLE_WARNING!>result !== path<!>) {}
+    if (result !== path) {}
 }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirEqualityCompatibilityChecker.kt

@@ -121,14 +121,12 @@ object FirEqualityCompatibilityChecker : FirEqualityOperatorCallChecker() {
         // we only need to check if one is related
         // to the other
 
-        fun TypeInfo.isSubclassOf(other: TypeInfo) = when {
-            other.enforcesEmptyIntersection -> type.classId == other.type.classId
-            else -> notNullType.isSubtypeOf(other.notNullType, context.session)
-        }
+        fun TypeInfo.isSubtypeOf(other: TypeInfo) =
+            notNullType.isSubtypeOf(other.notNullType, context.session)
 
         return when {
             l.type.isNothingOrNullableNothing || r.type.isNothingOrNullableNothing -> false
-            else -> !l.isSubclassOf(r) && !r.isSubclassOf(l)
+            else -> !l.isSubtypeOf(r) && !r.isSubtypeOf(l)
         }
     }
 

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.fir.kt

@@ -10,8 +10,8 @@ fun f(): Unit {
     x <!UNSAFE_OPERATOR_CALL!><<!> 1
     <!ASSIGNMENT_TYPE_MISMATCH!>x += 1<!>
 
-    <!EQUALITY_NOT_APPLICABLE_WARNING!>x == 1<!>
-    <!EQUALITY_NOT_APPLICABLE_WARNING!>x != 1<!>
+    x == 1
+    x != 1
 
     <!EQUALITY_NOT_APPLICABLE!>A() == 1<!>