IR: first somewhat meaningful expression IR generation

compiler/testData/ir/irText/boxOk.kt

@@ -0,0 +1,3 @@
+fun box(): String = "OK"
+
+// IR_FILE_TXT boxOk.txt

compiler/testData/ir/irText/boxOk.txt

@@ -0,0 +1,3 @@
+IrFile /boxOk.kt
+  IrFunction public fun box(): kotlin.String
+    ??? IrDummyBody

compiler/testData/ir/irText/smoke.kt

@@ -1,4 +1,4 @@
-fun testFun() {}
+fun testFun(): String { return "OK" }
 val testSimpleVal = 1
 val testValWithGetter: Int get() = 42
 var testSimpleVar = 2
@@ -10,8 +10,8 @@ var testVarWithAccessors: Int
 // 1 IrProperty public val testSimpleVal
 // 1 IrProperty public val testValWithGetter
 // 2 IrPropertyGetter
+// 1 IrPropertySetter
 // 1 IrProperty public var testSimpleVar
 // 1 IrProperty public var testVarWithAccessors
-// 1 IrPropertyGetter
 
 // IR_FILE_TXT smoke.txt

compiler/testData/ir/irText/smoke.txt

@@ -1,15 +1,21 @@
 IrFile /smoke.kt
-  IrFunction public fun testFun(): kotlin.Unit
-    ??? IrDummyBody
+  IrFunction public fun testFun(): kotlin.String
+    IrBlockExpression type=kotlin.Nothing
+      IrReturnExpression type=kotlin.Nothing
+        IrStringLiteral OK
   IrProperty public val testSimpleVal: kotlin.Int = 1
-    ??? IrDummyBody
+    IrReturnExpression type=kotlin.Int
+      IrIntLiteral type=kotlin.Int value=1
   IrProperty public val testValWithGetter: kotlin.Int
     IrPropertyGetter public fun <get-testValWithGetter>(): kotlin.Int
-      ??? IrDummyBody
+      IrReturnExpression type=kotlin.Int
+        IrIntLiteral type=kotlin.Int value=42
   IrProperty public var testSimpleVar: kotlin.Int
-    ??? IrDummyBody
+    IrReturnExpression type=kotlin.Int
+      IrIntLiteral type=kotlin.Int value=2
   IrProperty public var testVarWithAccessors: kotlin.Int
     IrPropertyGetter public fun <get-testVarWithAccessors>(): kotlin.Int
-      ??? IrDummyBody
+      IrReturnExpression type=kotlin.Int
+        IrIntLiteral type=kotlin.Int value=42
     IrPropertySetter public fun <set-testVarWithAccessors>(/*0*/ v: kotlin.Int): kotlin.Unit
-      ??? IrDummyBody
+      IrBlockExpression type=kotlin.Unit