patch 8.2.4669: in compiled code len('string') is not inlined

Problem:    In compiled code len('string') is not inlined.
Solution:   Compute the length at compile time if possible. (closes #10065)
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index e55e38e..f040ba6 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -1015,9 +1015,20 @@
   endif
 enddef
 
+def s:LenConstant(): number
+  return len("foo") + len("fighters")
+enddef
+
 def Test_disassemble_const_expr()
+  var instr = execute('disassemble LenConstant')
+  assert_match('LenConstant\_s*' ..
+    'return len("foo") + len("fighters")\_s*' ..
+    '\d PUSHNR 11\_s*',
+    instr)
+  assert_notmatch('BCALL len', instr)
+
   assert_equal("\nyes", execute('HasEval()'))
-  var instr = execute('disassemble HasEval')
+  instr = execute('disassemble HasEval')
   assert_match('HasEval\_s*' ..
         'if has("eval")\_s*' ..
         'echo "yes"\_s*' ..