FIR: Rework receivers processing in resolution

- Put extensionReceiver to candidate even if it's explicit (for sake of clarity)
- Split CheckReceiver (dispatch part should only check nullability)

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/extensionReceiver.fir.kt

@@ -17,7 +17,7 @@ fun CharSequence?.isNullOrEmpty(): Boolean {
 fun smartcastOnReceiver(s: String?) {
     with(s) {
         if (isNullOrEmpty()) {
-            length
+            <!INAPPLICABLE_CANDIDATE!>length<!>
         }
         else {
             length
@@ -32,7 +32,7 @@ fun mixedReceiver(s: String?) {
         }
     } else {
         with(s) {
-            length
+            <!INAPPLICABLE_CANDIDATE!>length<!>
         }
     }
-}
+}

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/extensionReceiver_after.fir.kt

@@ -17,7 +17,7 @@ fun CharSequence?.isNullOrEmpty(): Boolean {
 fun smartcastOnReceiver(s: String?) {
     with(s) {
         if (isNullOrEmpty()) {
-            length
+            <!INAPPLICABLE_CANDIDATE!>length<!>
         }
         else {
             length
@@ -32,7 +32,7 @@ fun mixedReceiver(s: String?) {
         }
     } else {
         with(s) {
-            length
+            <!INAPPLICABLE_CANDIDATE!>length<!>
         }
     }
-}
+}