[FIR] Completely unwrap original argument when computing type arguments

When computing the conversion type for a type operator call, the
argument needs to be fully unwrapped before getting the resolved type.
This avoids the situation when the argument is a when-subject and the
assumed original type is not correct. Before, only smart-casts were
unwrapped, and this change will also unwrap when-subjects (as well as a
few other FirExpressions).

^KT-62114 Fixed

compiler/testData/diagnostics/tests/smartCasts/implicitReceiver.kt

@@ -1,3 +1,5 @@
+// ISSUE: KT-62114
+
 open class A {
     class B : A() {
         val a = "FAIL"
@@ -27,3 +29,26 @@ class C {
         else return ""
     }
 }
+
+sealed class Received<out T> {
+    sealed class Error<out T> : Received<T>() {
+        data class SomeError<out T>(val details: T?) : Error<T>()
+    }
+}
+
+val Received<String>.thisRaisesUnresolvedReference: Boolean
+    get() = if (this is Received.Error<*>) {
+        when (<!DEBUG_INFO_SMARTCAST!>this<!>) {
+            is Received.Error.SomeError -> <!DEBUG_INFO_IMPLICIT_RECEIVER_SMARTCAST!>details<!>?.length == 0
+        }
+    } else {
+        false
+    }
+
+val Received<String>.thisIsFine: Boolean
+    get() = if (this is Received.Error<*>) {
+        if (this is Received.Error.SomeError) { <!DEBUG_INFO_IMPLICIT_RECEIVER_SMARTCAST!>details<!>?.length == 0 }
+        else false
+    } else {
+        false
+    }