FIR: Fix test data after making LHS of assignment an expression

KT-54648

compiler/testData/diagnostics/tests/cast/AsInCompoundAssignment.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+class A {
+    var b = 1
+}
+
+fun Any.test() {
+    (this as A).b += 1 <!USELESS_CAST!>as Int<!>
+}

compiler/testData/diagnostics/tests/cast/AsInCompoundAssignment.txt

@@ -0,0 +1,11 @@
+package
+
+public fun kotlin.Any.test(): kotlin.Unit
+
+public final class A {
+    public constructor A()
+    public final var b: kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}