Fix CFG for return instructions

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphShared.qll

@@ -432,9 +432,6 @@ module GoCfg {
         or
         n instanceof Go::IncDecStmt and tag = "incdec-rhs"
         or
-        // Return node
-        n instanceof Go::ReturnStmt and tag = "return"
-        or
         // Result write nodes in return statements
         exists(int i, Go::ReturnStmt ret |
           n = ret and
@@ -950,7 +947,7 @@ module GoCfg {
           n2.isAdditional(ret, tag2)
         )
         or
-        // Last return epilogue → In(ret) (the return itself)
+        // Last return epilogue → return node
         n1.isAdditional(ret, getLastReturnEpilogueTag(ret)) and
         n2.isIn(ret)
       )

go/ql/lib/semmle/go/controlflow/IR.qll

@@ -893,7 +893,7 @@ module IR {
   class ReturnInstruction extends Instruction {
     ReturnStmt ret;
 
-    ReturnInstruction() { this.isAdditional(ret, "return") }
+    ReturnInstruction() { this.isIn(ret) }
 
     /** Gets the corresponding `ReturnStmt`. */
     ReturnStmt getReturnStmt() { result = ret }
@@ -938,9 +938,7 @@ module IR {
       exists(retStmt.getAnExpr())
     }
 
-    private ReturnInstruction getReturnInstruction() {
-      result.(ReturnInstruction).isAdditional(retStmt, "return")
-    }
+    private ReturnInstruction getReturnInstruction() { result.getReturnStmt() = retStmt }
 
     override Instruction getRhs() { result = this.getReturnInstruction().getResult(idx) }