WASM: Stupidify unit handling

Now every IR instruction returns an actual Unit instance.
All Unit's are actual objects passed around except for function boundaries.

compiler/testData/codegen/box/unit/sillyThings.kt

@@ -0,0 +1,67 @@
+// Some silly things you can do with unit types.
+
+fun foo() {
+}
+
+fun zoot(): String {
+    return "str"
+}
+
+class Blumbs {
+    var t: Unit = Unit
+}
+
+fun varfoo(vararg t: Unit) {
+}
+
+fun foo1() {
+    if (zoot() == "str")
+        return
+    return
+}
+
+fun box(): String {
+    // Only check that this code can be compiled and verified.
+
+    1
+
+    {}
+
+    val tmp = Unit
+    {
+
+    }()
+
+    val tmp2 = {
+
+    }()
+
+    val tmp3 = Blumbs()
+    tmp3.t = Unit
+
+    val tmp4 = Blumbs()
+    tmp4.t = foo()
+
+    val units = arrayOf(Unit, Unit, Unit)
+    varfoo(Unit, *units, foo(), Unit)
+
+    val tmp5 = foo() as? Blumbs
+
+    val tmp6 = if (zoot() == "str") Unit else foo()
+
+    // From spec
+    val a /* : () -> Unit */ = {
+        if (true) 42
+        // CSB with no last expression
+        // Type is defined to be `kotlin.Unit`
+    }
+
+    val b: () -> Unit = {
+        if (true) 42 else -42
+        // CSB with last expression of type `kotlin.Int`
+        // Type is expected to be `kotlin.Unit`
+        // Coercion to kotlin.Unit applied
+    }
+
+    return "OK"
+}