[JVM_IR]: Improve stepping for when.

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

compiler/testData/debug/stepping/whenComplicatedSubject.kt

@@ -0,0 +1,49 @@
+// FILE: test.kt
+
+fun foo(x: Int) {
+   when (val y =
+           when {
+             x == 0 -> 1
+             x == 1 -> 2
+             else -> 0
+         }) {
+       0 -> 3
+       1 -> 4
+   }
+}
+
+fun box() {
+    foo(0)
+    foo(1)
+    foo(2)
+}
+
+// The JVM_IR backend optimizes the inner when to a switch and therefore goes directly to the
+// right case without stepping through evaluation of each condition.
+
+// LINENUMBERS
+// test.kt:16 box
+// test.kt:5 foo
+// test.kt:6 foo
+// test.kt:4 foo
+// test.kt:11 foo
+// test.kt:13 foo
+// test.kt:17 box
+// test.kt:5 foo
+// LINENUMBERS JVM
+// test.kt:6 foo
+// LINENUMBERS
+// test.kt:7 foo
+// test.kt:4 foo
+// test.kt:13 foo
+// test.kt:18 box
+// test.kt:5 foo
+// LINENUMBERS JVM
+// test.kt:6 foo
+// test.kt:7 foo
+// LINENUMBERS
+// test.kt:8 foo
+// test.kt:4 foo
+// test.kt:10 foo
+// test.kt:13 foo
+// test.kt:19 box