Testing framework: allow comparing subtrees starting at a given line with expected dump text.

compiler/testData/ir/irText/calls.txt

@@ -20,10 +20,10 @@ IrFile /calls.kt
   IrFunction public fun kotlin.Int.ext1(): kotlin.Int
     IrExpressionBody
       RETURN type=kotlin.Int
-        DUMMY KtThisExpression
+        $RECEIVER of: <root>
   IrFunction public fun kotlin.Int.ext2(/*0*/ x: kotlin.Int): kotlin.Int
     IrExpressionBody
       RETURN type=kotlin.Int
         CALL foo
-          x: DUMMY KtThisExpression
+          x: $RECEIVER of: <root>
           y: VAR x

compiler/testData/ir/irText/smoke.kt

@@ -1,5 +1,5 @@
 fun testFun(): String { return "OK" }
-val testSimpleVal = 1
+val testSimpleVal = 1 // <<< smoke.testSimpleVal.txt
 val testValWithGetter: Int get() = 42
 var testSimpleVar = 2
 var testVarWithAccessors: Int

compiler/testData/ir/irText/smoke.testSimpleVal.txt

@@ -0,0 +1,4 @@
+IrProperty public val testSimpleVal: kotlin.Int = 1 getter=null setter=null
+  IrExpressionBody
+    RETURN type=kotlin.Int
+      LITERAL Int type=kotlin.Int value='1'