More precise diagnostics of smart cast impossible #KT-7240 Fixed

compiler/testData/diagnostics/testsWithStdLib/smartcasts/forEachSafe.kt

@@ -3,7 +3,7 @@
 fun indexOfMax(a: IntArray): Int? {
     var maxI: Int? = null
     a.forEachIndexed { i, value ->
-        if (maxI == null || value >= a[<!TYPE_MISMATCH!>maxI<!>]) {
+        if (maxI == null || value >= a[<!SMARTCAST_IMPOSSIBLE!>maxI<!>]) {
             maxI = i
         }
     }

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letChangesToNull.kt

@@ -4,6 +4,6 @@ fun foo(y: String?) {
     var x: String? = ""
     if (x != null) {
         y?.let { x = null }
-        x<!UNSAFE_CALL!>.<!>length // Smart cast is not possible
+        <!SMARTCAST_IMPOSSIBLE!>x<!>.length // Smart cast is not possible
     }
 }

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letChangesToNullComplex.kt

@@ -6,6 +6,6 @@ fun foo(y: String?) {
     var x: String? = ""
     if (x != null) {
         bar(y?.let { x = null; it })<!UNSAFE_CALL!>.<!>length
-        x<!UNSAFE_CALL!>.<!>length // Smart cast is not possible
+        <!SMARTCAST_IMPOSSIBLE!>x<!>.length // Smart cast is not possible
     }
 }

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letMergeNotNull.kt

@@ -4,6 +4,6 @@ fun foo(y: String?) {
     var x: String? = null
     if (x != null) {
         y?.let { x = it }
-        x<!UNSAFE_CALL!>.<!>length // not-null or not-null
+        <!SMARTCAST_IMPOSSIBLE!>x<!>.length // not-null or not-null
     }
 }

compiler/testData/diagnostics/testsWithStdLib/smartcasts/withChangesToNull.kt

@@ -3,8 +3,8 @@ fun foo(y: String?) {
     if (x != null) {
         with(y?.let { x = null; it }) {
             this<!UNSAFE_CALL!>.<!>length
-            x<!UNSAFE_CALL!>.<!>length
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.length
         }
-        x<!UNSAFE_CALL!>.<!>length
+        <!SMARTCAST_IMPOSSIBLE!>x<!>.length
     }
 }