FIR: fork flow on function entry, restore receivers on exit

^KT-52680 Fixed

compiler/testData/diagnostics/tests/smartCasts/loops/doWhileLiteral.fir.kt

@@ -1,10 +0,0 @@
-fun x(): Boolean { return true }
-
-public fun foo(p: String?): Int {
-    // Exotic variant with unused literal
-    do { ->
-        p!!.length
-    } while (!x())
-    // Literal is not called so p.length is unsafe
-    return p.length
-}

compiler/testData/diagnostics/tests/smartCasts/loops/doWhileLiteral.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun x(): Boolean { return true }
 
 public fun foo(p: String?): Int {