Fix flat-switch for Go code

In flat-switch mode switches without a default case add an alternative edge from the head to the merge.

Author: tmr232

src/control-flow/cfg-go.ts

@@ -159,17 +159,17 @@ export class CFGBuilder {
     switchHeadNode: string,
   ) {
     let fallthrough: string | null = null;
-    let previous: string | null = null;
-    if (!this.flatSwitch && cases[0]?.conditionEntry) {
-      this.addEdge(switchHeadNode, cases[0].conditionEntry);
-    }
+    let previous: string | null = switchHeadNode;
     cases.forEach((thisCase) => {
       if (this.flatSwitch) {
         if (thisCase.consequenceEntry) {
           this.addEdge(switchHeadNode, thisCase.consequenceEntry);
           if (fallthrough) {
             this.addEdge(fallthrough, thisCase.consequenceEntry);
           }
+          if (thisCase.isDefault) {
+            previous = null;
+          }
         }
       } else {
         if (fallthrough && thisCase.consequenceEntry) {
@@ -203,7 +203,6 @@ export class CFGBuilder {
     });
     // Connect the last node to the merge node.
     // No need to handle `fallthrough` here as it is not allowed for the last case.
-    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     if (previous) {
       this.addEdge(previous, mergeNode, "alternative");
     }