Avoid jumping to a wrong destination

when the next insn is already compiled by former branches.

test/ruby/test_jit.rb

@@ -828,6 +828,16 @@ class TestJIT < Test::Unit::TestCase
     end;
   end
 
+  def test_jump_to_precompiled_branch
+    assert_eval_with_jit("#{<<~'begin;'}\n#{<<~'end;'}", stdout: ".0", success_count: 1, min_calls: 1)
+    begin;
+      def test(foo)
+        ".#{foo unless foo == 1}" if true
+      end
+      print test(0)
+    end;
+  end
+
   def test_clean_so
     if RUBY_PLATFORM.match?(/mswin/)
       skip 'Removing so file is randomly failing on AppVeyor/RubyCI mswin due to Permission Denied.'

tool/ruby_vm/views/_mjit_compile_insn.erb

@@ -76,9 +76,14 @@
     }
     fprintf(f, "}\n");
 %
-% # compiler: If insn has conditional JUMP, the branch which is not targeted by JUMP should be compiled too.
+% # compiler: If insn has conditional JUMP, the code should go to the branch not targeted by JUMP next.
 % if insn.expr.expr =~ /if\s+\([^{}]+\)\s+\{[^{}]+JUMP\([^)]+\);[^{}]+\}/
-    compile_insns(f, body, b->stack_size, pos + insn_len(insn), status);
+    if (ALREADY_COMPILED_P(status, pos + insn_len(insn))) {
+        fprintf(f, "goto label_%d;\n", pos + insn_len(insn));
+    }
+    else {
+        compile_insns(f, body, b->stack_size, pos + insn_len(insn), status);
+    }
 % end
 %
 % # compiler: If insn returns (leave) or does longjmp (throw), the branch should no longer be compiled. TODO: create attr for it?