Receiver of a safe call is not null inside the call also for extension functions + a pair of tests #KT-9033 Fixed

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

@@ -0,0 +1,7 @@
+// Based on KT-9033
+fun f(s: String) = s
+
+fun foo(s: String?) {
+    s?.let { f(<!DEBUG_INFO_SMARTCAST!>s<!>) }
+    s?.let { f(it) }
+}

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letUsesOwnReceiver.txt

@@ -0,0 +1,4 @@
+package
+
+public fun f(/*0*/ s: kotlin.String): kotlin.String
+public fun foo(/*0*/ s: kotlin.String?): kotlin.Unit