[llvm] Don't rely on undefined behavior to store how a `User` object's allocation is laid out (PR #105714)

[Daniel Paoliello via llvm-commits]

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@@ -454,6 +453,9 @@ GlobalVariable::GlobalVariable(Type *Ty, bool constant, LinkageTypes Link,
     assert(InitVal->getType() == Ty &&
            "Initializer should be the same type as the GlobalVariable!");
     Op<0>() = InitVal;
+  } else {
+    // HACK: This is incorrect, we should have allocated less memory.
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dpaoliello wrote:

Ah, I didn't see that `operator delete` fixed this!

I moved the fixup into the destructor, and moved `setGlobalVariableNumOperands` into `GlobalVariable` and made it private (as it's only used in `GlobalVariable`).

I also removed my comment in the ctor as I'm now calling `setGlobalVariableNumOperands` and that (and the dtor) have comments to explain what's happening.

https://github.com/llvm/llvm-project/pull/105714