Accessing deeply nested variable at 'if' statemet cause type collision

還沒有人認領這個 Issue。

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Type Checking

### Expected Behaviour

Type checking does not breaks after accessing variable at 'if' statement.

### Actual Behaviour

2nd nested variable type became '\ | unknown' so accessing variable or function which not exists in this class does not cause warning.

### Reproduction steps

```lua
---@class DemoD
---@field c integer
local DemoD = {}
DemoD.c = 0

---@class DemoC
local DemoC = {}
DemoC.D = DemoD

---@class DemoB
local DemoB = {}
DemoB.C = DemoC

---@class DemoA
local DemoA = {}
DemoA.B = DemoB

---@class DemoRoot
DemoRoot = {}
DemoRoot.A = DemoA

local d = DemoRoot.A.B.C.D

-- if d.c > 1 then end -- if we replace 'DemoRoot.A.B.C.D' with 'd' then F is undefined in all 4 cases
if DemoRoot.A.B.C.D.c > 1 then end

DemoRoot.F() -- warning: F is undefined
DemoRoot.A.F() -- warning: F is undefined
DemoRoot.A.B.F() -- no warning
DemoRoot.A.B.C.F() -- no warning
```

### Additional Notes

_No response_

### Log File

_No response_

首先，使用 issue 中的 Lua 程式碼片段重現回報的行為，並比較 Visual Studio Code 擴充功能所使用的型別檢查器中的直接形式與區域變數形式。當 DemoRoot.A.B.F() 和 DemoRoot.A.B.C.F() 等無效成員產生未定義成員警告，同時不破壞有效的巢狀存取時，修正才算完成。

由索引模型根據 Issue 內容生成。