Fix op asgn method calls passing mutable keyword splats

When passing the keyword splat to [], it cannot be mutable, because
mutating the keyword splat inside [] would result in changes to the
keyword splat passed to []=.

compile.c

@@ -8908,7 +8908,7 @@ compile_op_asgn1(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node
     }
     ADD_INSN1(ret, node, dupn, INT2FIX(dup_argn));
     flag |= asgnflag;
-    ADD_SEND_R(ret, node, idAREF, argc, NULL, INT2FIX(flag), keywords);
+    ADD_SEND_R(ret, node, idAREF, argc, NULL, INT2FIX(flag & ~VM_CALL_KW_SPLAT_MUT), keywords);
 
     if (id == idOROP || id == idANDOP) {
         /* a[x] ||= y  or  a[x] &&= y

test/ruby/test_call.rb

@@ -281,6 +281,27 @@ def o.[]=(...) end
     assert_equal([:to_a, :to_hash, :to_proc, :to_proc], ary)
   end
 
+  def test_call_op_asgn_keywords_mutable
+    h = Class.new do
+      attr_reader :get, :set
+      def v; yield; [*@get, *@set] end
+      def [](*a, **b, &c)
+        @get = [a.dup, b.dup]
+        a << :splat_modified
+        b[:kw_splat_modified] = true
+        @set = []
+        3
+      end
+      def []=(*a, **b) @set = [a, b] end
+    end.new
+
+    a = []
+    kw = {}
+    b = lambda{}
+
+    assert_equal([[2], {b: 5}, [2, 4], {b: 5}], h.v{h[*a, 2, b: 5, **kw] += 1})
+  end
+
   def test_call_splat_order
     bug12860 = '[ruby-core:77701] [Bug# 12860]'
     ary = [1, 2]