`repeat…until val ~= nil` should satisfy the need for nil checks

還沒有人認領這個 Issue。

I have some code where I have to wait for a forked process to create a file and I can't get feedback over its return status or stdout, so I use a polling pattern similar to this:

```lua
local val = nil

-- kick off the external process
fork_process(...)

repeat
val = check_for_val(...)
until val ~= nil

val:foo()
```

In this current form, I get a diagnostic that says I need to perform a nil check on `val` before calling `foo`, but the loop condition here should be considered the nil check.

This already works for `while val == nil do...end` so it would be nice to see it for `repeat...until val~= nil` as well.

該 issue 未指定原始檔案或測試；先找出處理 while 迴圈和 repeat…until 條件之 nil 檢查的 flow analysis 邏輯。重現 polling 範例，並將其診斷結果與現有的 while val == nil 行為進行比較；當 repeat…until val ~= nil 之後，val:foo() 不再需要額外的 nil 檢查時，即表示完成。

由索引模型根據 Issue 內容生成。