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_cmdline.vim b/src/testdir/test_cmdline.vim
index 3f73d3d..7a4979e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -184,6 +184,7 @@
   endif
   for cmd in [
 	\ 'let a = ',
+	\ 'const a = ',
 	\ 'if',
 	\ 'elseif',
 	\ 'while',
diff --git a/src/testdir/test_const.vim b/src/testdir/test_const.vim
index 55e4492..8df34f4 100644
--- a/src/testdir/test_const.vim
+++ b/src/testdir/test_const.vim
@@ -197,6 +197,12 @@
     call assert_fails('const [i2, f2, s2] = [1, 1.1, "vim"]', 'E995:')
 endfunc
 
+func Test_const_with_condition()
+  const x = 0
+  if 0 | const x = 1 | endif
+  call assert_equal(0, x)
+endfunc
+
 func Test_const_with_index_access()
     let l = [1, 2, 3]
     call assert_fails('const l[0] = 4', 'E996:')
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