patch 8.1.2348: :const cannot be followed by "| endif"

Problem:    :const cannot be followed by "| endif".
Solution:   Check following command for :const. (closes #5269)
            Also fix completion after :const.
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 4d9aae9..055425d 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -24,6 +24,10 @@
   let out = execute('let a {0 == 1 ? "a" : "b"}')
   let s = "\na                     #1\nb                     #2"
   call assert_equal(s, out)
+
+  let x = 0
+  if 0 | let x = 1 | endif
+  call assert_equal(0, x)
 endfunc
 
 func s:set_arg1(a) abort