[lld] [lld][ELF] Merge equivalent symbols found during ICF (PR #139493)

[Fangrui Song via llvm-commits]

================
@@ -286,9 +313,14 @@ bool ICF<ELFT>::constantEq(const InputSection *secA, Relocs<RelTy> ra,
     // Relocations referring to InputSections are constant-equal if their
     // section offsets are equal.
     if (isa<InputSection>(da->section)) {
-      if (da->value + addA == db->value + addB)
+      if (da->value + addA == db->value + addB) {
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MaskRay wrote:

This change was not in #134342 . I have tried to understand why the logic is so complex.
Looks like https://reviews.llvm.org/D34094 , while supporting MergeInputSection, also introduced the untested condition for InputSection
```
    if (isa<InputSection>(DA->Section))
      return DA->Value + AddA == DB->Value + AddB;
```

I feel that we should switch to the safer `da->value == db->value && addA == addB`, which likely doesn't lose anything (I believe it's difficult for clang emitted code to utilize the relaxed condition.)

https://github.com/llvm/llvm-project/pull/139493