Preserve wasm pointer allocas for shadow stack GC under -O2

[Copilot]

Optimized WebAssembly codegen can drop shadow stack synchronization by promoting pointer allocas into locals, leaving GC roots invisible in linear memory.

• Problem summary: O2 optimizations (e.g., mem2reg/localization) can eliminate stack writes for pointer roots, breaking shadow-stack style GC visibility.
• Changes: Prevent mem2reg from promoting pointer allocas on wasm targets to keep roots in linear memory, and add a WebAssembly codegen test to ensure a store remains before a GC call.

Original prompt

目前我的研究发现，使用默认优化时，llvm会保留将local值写回C栈（或者在wasm的语义下叫作Shadow Stack, 即以__stack__pointer为栈底指针的linear memory区域）的功能（可能对于函数传参的情况不完整？我不确定）；在O2优化之后，同步机制就被破坏了，这是由于llvm里的一些优化，如mem2reg，破坏了一些东西，导致： 1. 可能是维护shadow stack的llvm ir缺失了 2. 可能是最后wasm生成器，无法从优化后的llvm ir里识别到，某一处需要插一个维护shadow stack的操作 这些功能将Shadow Stack同步功能破坏掉，导致无法正确写回，进而导致boehm gc失效。我现在希望能够在O2优化下也保留这个功能。请为我实现这个功能，或者至少给我定位到导致同步失效的代码块及原因。
可以参考仓库：WebAssembly/binaryen仓库的SpillPointer.cpp。
目标仓库：PLUTOFX/llvm-project

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[PLUTOFX]

@copilot I got it. Then implement this fix and check this fix.

[PLUTOFX]

@copilot 实际上我的问题是，一个变量通过堆分配拿到的指针，只存在local寄存器里，没往linear memory的shadow stack（即(__stack_pointer + n)的内存区域)里同步。并不是在local里寄存的__stack_pointer本身没有向global同步。举例：
对于函数cons：
// 链表节点结构体
typedef struct Node {
int value;
struct Node* next;
} Node;

// 创建新节点并添加到链表头部
Node* cons(int x, Node* xs) {
Node* n = GC_malloc(sizeof(Node));
n->value = x;
n->next = xs;
GC_gcollect();
return n;
}

下面是O0情况下的被编译成的wasm指令，可以看到新的node被存储到了__stack_pointer + n的区域
(func $cons (type 8) (param i32 i32) (result i32)
(local i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32 i32)
global.get $__stack_pointer
local.set 2
i32.const 16
local.set 3
local.get 2
local.get 3
i32.sub
local.set 4
local.get 4
global.set $__stack_pointer
local.get 4
local.get 0
i32.store offset=12
local.get 4
local.get 1
i32.store offset=8
i32.const 8
local.set 5
local.get 5
call $GC_malloc
local.set 6
local.get 4
local.get 6
i32.store offset=4
local.get 4
i32.load offset=12
local.set 7
local.get 4
i32.load offset=4
local.set 8
local.get 8
local.get 7
i32.store
local.get 4
i32.load offset=8
local.set 9
local.get 4
i32.load offset=4
local.set 10
local.get 10
local.get 9
i32.store offset=4
call $GC_gcollect
local.get 4
i32.load offset=4
local.set 11
i32.const 16
local.set 12
local.get 4
local.get 12
i32.add
local.set 13
local.get 13
global.set $__stack_pointer
local.get 11
return)
但是在O2优化下，分配的局部变量不会被同步到stack区域内，而只是存储在local里。
请结合我的新说明，再次进行研究。

[PLUTOFX]

@copilot I got it. Then implement this fix and check this fix.

[Copilot]

@copilot I got it. Then implement this fix and check this fix.

Implemented a fix to keep wasm pointer allocas from being promoted by mem2reg so values stay in linear memory for shadow-stack style GC, plus added a WebAssembly codegen test. Commit: 2dcd536. No UI changes to screenshot.