chore: Add panic for compiler error described in #6620 (#6621)

compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs

@@ -432,6 +432,16 @@ impl<'f> Context<'f> {
         };
         self.condition_stack.push(cond_context);
         self.insert_current_side_effects_enabled();
+
+        // We disallow this case as it results in the `else_destination` block
+        // being inlined before the `then_destination` block due to block deduplication in the work queue.
+        //
+        // The `else_destination` block then gets treated as if it were the `then_destination` block
+        // and has the incorrect condition applied to it.
+        assert_ne!(
+            self.branch_ends[if_entry], *then_destination,
+            "ICE: branches merge inside of `then` branch"
+        );
         vec![self.branch_ends[if_entry], *else_destination, *then_destination]
     }
 
@@ -1476,4 +1486,23 @@ mod test {
             _ => unreachable!("Should have terminator instruction"),
         }
     }
+
+    #[test]
+    #[should_panic = "ICE: branches merge inside of `then` branch"]
+    fn panics_if_branches_merge_within_then_branch() {
+        //! This is a regression test for https://github.com/noir-lang/noir/issues/6620
+
+        let src = "
+        acir(inline) fn main f0 {
+          b0(v0: u1):
+            jmpif v0 then: b2, else: b1
+          b2():
+            return
+          b1():
+            jmp b2()           
+        }
+        ";
+        let merged_ssa = Ssa::from_str(src).unwrap();
+        let _ = merged_ssa.flatten_cfg();
+    }
 }