Fixed self value in a block is changed with return value

fix #1504

src/codegen.c

@@ -286,8 +286,7 @@ genop_peep(codegen_scope *s, mrb_code i, int val)
       case OP_RANGE:
       case OP_AREF:
       case OP_GETUPVAR:
-        s->iseq[s->pc-1] = MKOP_ABC(c0, 0, GETARG_B(i0), GETARG_C(i0));
-        genop(s, MKOP_AB(OP_RETURN, 0, OP_R_NORMAL));
+        genop(s, MKOP_AB(OP_RETURN, GETARG_A(i0), OP_R_NORMAL));
         return;
       case OP_SETIV:
       case OP_SETCV:

test/t/proc.rb

@@ -54,3 +54,19 @@ assert('Proc#call', '15.2.17.4.3') do
   assert_equal 1, a
   assert_equal 5, a2
 end
+
+assert('Proc#return_does_not_break_self') do
+  class TestClass
+    attr_accessor :block
+    def initialize
+    end
+    def register_block
+      @block = Proc.new { self }
+      return []
+    end
+  end
+
+  c = TestClass.new
+  assert_equal [], c.register_block
+  assert_equal c, c.block.call
+end