JVM_IR don't inline temporary val for 'when' subject

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/JvmOptimizationLowering.kt

@@ -393,6 +393,14 @@ class JvmOptimizationLowering(val context: JvmBackendContext) : FileLoweringPass
                 dontTouchTemporaryVals.add(safeCall.tmpVal)
             }
 
+            if (expression.origin == IrStatementOrigin.WHEN) {
+                // Don't optimize out 'when' subject - otherwise we might get somewhat weird debugging behavior.
+                val subject = expression.statements.firstOrNull()
+                if (subject is IrVariable) {
+                    dontTouchTemporaryVals.add(subject)
+                }
+            }
+
             expression.transformChildren(this, data)
             removeUnnecessaryTemporaryVariables(expression.statements)
             return expression

compiler/testData/debug/stepping/whenConstant.kt

@@ -12,14 +12,9 @@ fun box() {
 }
 
 // JVM_IR and JVM backends have different heuristics for when to use a switch.
-// JVM_IR does not use a switch in this case and therefore steps to the evaluation
-// of the condition for each of the cases.
+// JVM_IR inlines constant value subject and folds unreachable branches.
 
 // LINENUMBERS
 // test.kt:4 box
-// LINENUMBERS JVM_IR
-// test.kt:5 box
-// test.kt:7 box
-// LINENUMBERS
 // test.kt:10 box
 // test.kt:12 box