JVM_IR: Fix stack underflow for unit coercion of when expressions.

Code such as

```
val b = getBoolean()
if (b) 4
else if (b) 5
```

didn't generate a value on the stack always and therefore would
have control-flow paths leading to a pop instruction with nothing
on the stack.

Change-Id: I09d059f361e56a41880006e3f4e51e9acdbd167d

compiler/testData/codegen/box/when/noElseCoerceToUnit.kt

@@ -0,0 +1,22 @@
+var result = "FAIL"
+val d = 0.0
+
+fun test(arg: Int) {
+    if (arg == 1) {
+        result = "firstResult"
+        d
+    } else if (arg == 2) {
+        result = "secondResult"
+        arg
+    }
+}
+
+fun box(): String {
+    test(1)
+    if (result != "firstResult")
+        return "FAIL1"
+    test(2)
+    if (result != "secondResult")
+        return "FAIL2"
+    return "OK"
+}