Basic class members generation.

Split testData into 'classes' and 'expressions'.

compiler/testData/ir/irText/expressions/elvis.kt

@@ -0,0 +1,17 @@
+val p: Any? = null
+
+fun foo(): Any? = null
+
+fun test1(a: Any?, b: Any) = a ?: b
+
+fun test2(a: String?, b: Any) = a ?: b
+
+fun test3(a: Any?, b: Any?): String {
+    if (b !is String) return ""
+    if (a !is String?) return ""
+    return a ?: b
+}
+
+fun test4(x: Any) = p ?: x
+
+fun test5(x: Any) = foo() ?: x