patch 9.0.1973: Clean up cmdline option completion code

Problem:  Clean up cmdline option completion code
Solution: Fix various minor problems

- Fix manual array size calculations to just use `ARRAY_LENGTH()`.
- Fix unintentional typo in comments due to copy-paste error.
- Fix assert_equal() usages to pass the expected value to first
  parameter instead of 2nd one to avoid confusion.
- Fix signed vs unsigned warnings
- Correct misplaced comments about set_op_T and set_prefix_T
  and fix a typo in another comment

closes: #13249
closes: #13237

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/structs.h b/src/structs.h
index 1508f47..8443a42 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -588,6 +588,9 @@
     XP_PREFIX_INV,	// "inv" prefix for bool option
 } xp_prefix_T;
 
+/*
+ * :set operator types
+ */
 typedef enum {
     OP_NONE = 0,
     OP_ADDING,		// "opt+=arg"