KT-16437 Incorrect type inference for some when coerced to Unit

If the result of 'when' is not used in an expression,
this 'when' expression has type 'Unit' despite of whatever FE has inferred.

compiler/testData/ir/irText/expressions/whenCoercedToUnit.kt

@@ -0,0 +1,7 @@
+// WITH_RUNTIME
+
+fun foo(x: Int) {
+    when (x) {
+        0 -> 0
+    }
+}