[llvm] [GlobalOpt] Don't replace aliasee with alias that has weak linkage (PR #91483)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Ah, I misread the code.
In `bar`, `call void @f1()` calling the aliasee was replaced with the alias before (`RenameTarget`).
This was problematic and this patch have fixed it.

This patch and its backport to release/18.x looks good to me.

I agree that some cleanup could be made to improve readability of this code.

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> At -O0, the initialization of "a" and "b" happens in define internal void @__tls_init(). At -Oz, this becomes linkonce_odr dso_local void @_ZTH1a()... so if a is also defined in another file, we can skip the constructor for b.

Thanks for the example.

At -O0, `define internal void @__tls_init()` initializes `a` and `b`.
Previously, `__tls_init` was incorrectly renamed (due to `RenameTarget`) to `_ZTH1a` (like derefinement).
If the linker picks the prevailing definition of `_ZTH1a` from another module, we would skip the constructor of `b`.


https://github.com/llvm/llvm-project/pull/91483