[FIR] Move FirReturnsImpliesAnalyzer to the extended checkers

compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/kt45243.fir.kt

@@ -1,19 +0,0 @@
-// ISSUE: KT-45243
-
-import kotlin.contracts.ExperimentalContracts
-import kotlin.contracts.contract
-
-@ExperimentalContracts
-fun <T> assertNotNull(actual: T) {
-    <!WRONG_IMPLIES_CONDITION!>contract { returns() implies (actual != null) }<!>
-}
-
-@ExperimentalContracts
-fun test_1() {
-    assertNotNull { }
-}
-
-@ExperimentalContracts
-fun test_2() {
-    assertNotNull({ })
-}

compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/kt45243.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // ISSUE: KT-45243
 
 import kotlin.contracts.ExperimentalContracts