patch 9.1.1101: insexpand.c hard to read

patch 9.1.1101: insexpand.c hard to read

Problem:  insexpand.c hard to read
Solution: refactor slightly to make it better readable
          (glepnir)

Problem:
- Complex while loops with nested conditions
- Redundant if branches
- Hard to understand and maintain

Solution:
- Restructure using while(true) with clear break conditions
- Using ternary to replace some if conditions
- Add descriptive comments for each step

closes: #16600

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>