[JVM_IR]: Improve stepping for when.

Additionally, use the line number of the class for default interface
dispatch methods.

compiler/testData/debug/stepping/stringSwitchesSmall.kt

@@ -0,0 +1,96 @@
+// FILE: test.kt
+
+fun stringSwitch(x: String) {
+    val l = when {
+        x == "x" -> 1
+        x == "xy" -> 2
+        else -> -1
+    }
+
+    val l2 = when (x) {
+        "x" -> 1
+        "xy" -> 2
+        else -> -1
+    }
+
+    val l3 = when
+        (x)
+    {
+        "x" -> 1
+        "xy" -> 2
+        else -> -1
+    }
+}
+
+fun box() {
+    stringSwitch("x")
+    stringSwitch("xy")
+    stringSwitch("nope")
+}
+
+// JVM_IR uses the line number of the start of the `when` as the line number
+// for the lookup/table switch. Therefore when the subject and the when is
+// on separate lines the first step is on the subject, then steop to the when,
+// then to the right branch.
+
+// JVM_IR uses unoptimized lookup/table switches for all these cases. JVM
+// does not. So on JVM there are direct jumps to the right branch for the
+// last two whens.
+
+// LINENUMBERS
+// test.kt:26 box
+// test.kt:4 stringSwitch
+// test.kt:5 stringSwitch
+// test.kt:4 stringSwitch
+// test.kt:10 stringSwitch
+// test.kt:11 stringSwitch
+// test.kt:10 stringSwitch
+// test.kt:17 stringSwitch
+// LINENUMBERS JVM_IR
+// test.kt:16 stringSwitch
+// LINENUMBERS
+// test.kt:19 stringSwitch
+// test.kt:16 stringSwitch
+// test.kt:23 stringSwitch
+// test.kt:27 box
+// test.kt:4 stringSwitch
+// test.kt:5 stringSwitch
+// test.kt:6 stringSwitch
+// test.kt:4 stringSwitch
+// test.kt:10 stringSwitch
+// LINENUMBERS JVM_IR
+// test.kt:11 stringSwitch
+// LINENUMBERS
+// test.kt:12 stringSwitch
+// test.kt:10 stringSwitch
+// test.kt:17 stringSwitch
+// LINENUMBERS JVM_IR
+// test.kt:16 stringSwitch
+// test.kt:19 stringSwitch
+// LINENUMBERS
+// test.kt:20 stringSwitch
+// test.kt:16 stringSwitch
+// test.kt:23 stringSwitch
+// test.kt:28 box
+// test.kt:4 stringSwitch
+// test.kt:5 stringSwitch
+// test.kt:6 stringSwitch
+// test.kt:7 stringSwitch
+// test.kt:4 stringSwitch
+// test.kt:10 stringSwitch
+// LINENUMBERS JVM_IR
+// test.kt:11 stringSwitch
+// test.kt:12 stringSwitch
+// LINENUMBERS
+// test.kt:13 stringSwitch
+// test.kt:10 stringSwitch
+// test.kt:17 stringSwitch
+// LINENUMBERS JVM_IR
+// test.kt:16 stringSwitch
+// test.kt:19 stringSwitch
+// test.kt:20 stringSwitch
+// LINENUMBERS
+// test.kt:21 stringSwitch
+// test.kt:16 stringSwitch
+// test.kt:23 stringSwitch
+// test.kt:29 box