IR test tests: use that magical '<<<' everywhere. No need for 'IR_FILE_TXT'.

compiler/testData/ir/irText/boxOk.kt

@@ -1,3 +1,2 @@
+// <<< boxOk.txt
 fun box(): String = "OK"
-
-// IR_FILE_TXT boxOk.txt

compiler/testData/ir/irText/calls.kt

@@ -1,8 +1,7 @@
+// <<< calls.txt
 fun foo(x: Int, y: Int) = x
 fun bar(x: Int) = foo(x, 1)
 fun qux(x: Int) = foo(foo(x, x), x)
 
 fun Int.ext1() = this
 fun Int.ext2(x: Int) = foo(this, x)
-
-// IR_FILE_TXT calls.txt

compiler/testData/ir/irText/dotQualified.kt

@@ -1,4 +1,3 @@
+// <<< dotQualified.txt
 fun length(s: String) = s.length
 fun lengthN(s: String?) = s?.length
-
-// IR_FILE_TXT dotQualified.txt

compiler/testData/ir/irText/extensionPropertyGetterCall.kt

@@ -1,4 +1,3 @@
+// <<< extensionPropertyGetterCall.txt
 val String.okext: String get() = "OK"
 fun String.test5() = okext
-
-// IR_FILE_TXT extensionPropertyGetterCall.txt

compiler/testData/ir/irText/references.kt

@@ -1,3 +1,4 @@
+// <<< references.txt
 val ok = "OK"
 val ok2 = ok
 val ok3: String get() = "OK"
@@ -15,5 +16,3 @@ fun test4() = ok3
 
 val String.okext: String get() = "OK"
 fun String.test5() = okext
-
-// IR_FILE_TXT references.txt

compiler/testData/ir/irText/smoke.kt

@@ -1,3 +1,4 @@
+// <<< smoke.txt
 fun testFun(): String { return "OK" }
 val testSimpleVal = 1 // <<< smoke.testSimpleVal.txt
 val testValWithGetter: Int get() = 42
@@ -13,5 +14,3 @@ var testVarWithAccessors: Int
 // 1 IrPropertySetter
 // 1 IrProperty public var testSimpleVar
 // 1 IrProperty public var testVarWithAccessors
-
-// IR_FILE_TXT smoke.txt