Coroutines: Fix RedundantLocalsEliminationMethodTransformer

- Take control flow into account when collecting usage information
- Don't remove stores to local variables

compiler/testData/codegen/box/coroutines/unitTypeReturn/inlineUnitFunction.kt

@@ -0,0 +1,7 @@
+suspend fun f(x: Any?) {
+    x?.let { Unit } ?: Unit
+}
+
+fun box(): String {
+    return "OK"
+}