[JVM_IR] Give temporary variable loads meaningful offsets.

Temporary variable loads in when expressions had the offsets
of the variable declaration. That leads to hacks during codegen
for line number generation.

Instead of those hacks, give the variable loads the offsets of
the context in which they occur. That avoids the codegen hacks
and fixes stepping behavior for more when expressions.

compiler/testData/debug/stepping/whenIsChecks.kt

@@ -0,0 +1,38 @@
+// FILE: test.kt
+
+fun foo(x: Any) {
+    when (x) {
+        is Float ->
+            1
+        is Double ->
+            2
+        else ->
+            3
+    }
+}
+
+fun box() {
+    foo(1.2f)
+    foo(1.2)
+    foo(1)
+}
+
+// LINENUMBERS
+// test.kt:15 box
+// test.kt:4 foo
+// test.kt:5 foo
+// test.kt:6 foo
+// test.kt:12 foo
+// test.kt:16 box
+// test.kt:4 foo
+// test.kt:5 foo
+// test.kt:7 foo
+// test.kt:8 foo
+// test.kt:12 foo
+// test.kt:17 box
+// test.kt:4 foo
+// test.kt:5 foo
+// test.kt:7 foo
+// test.kt:10 foo
+// test.kt:12 foo
+// test.kt:18 box

compiler/testData/debug/stepping/whenNullalbeSubject.kt

@@ -0,0 +1,24 @@
+// FILE: test.kt
+
+fun box() {
+    val a: Int? = 0
+    when (a) {
+        1 -> {
+            1 + 1
+        }
+        2 -> {
+            2 + 1
+        }
+        else -> {
+            0 + 0
+        }
+    }
+}
+
+// LINENUMBERS
+// test.kt:4 box
+// test.kt:5 box
+// test.kt:6 box
+// test.kt:9 box
+// test.kt:13 box
+// test.kt:16 box