patch 8.2.4844: <C-S-I> is simplified to <S-Tab>

Problem:    <C-S-I> is simplified to <S-Tab>.
Solution:   Do not simplify CTRL if there is also SHIFT. (closes #10313)
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index 4ae01b8..96723b6 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -1560,4 +1560,10 @@
   bw!
 endfunc
 
+func Test_gui_CTRL_V()
+  call feedkeys(":let g:str = '\<C-V>\<*C-S-I>\<C-V>\<*C-S-@>'\<CR>", 'tx')
+  call assert_equal('<C-S-I><C-S-@>', g:str)
+  unlet g:str
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab