[llvm] [ConstraintElim] decompose subtract with guard as precondition (PR #209615)

[Antonio Frighetto via llvm-commits]

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@@ -664,6 +664,15 @@ static Decomposition decompose(Value *V,
     return V;
   }
 
+  if (match(V, m_Sub(m_Value(Op0), m_Value(Op1)))) {
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antoniofrighetto wrote:

May as well refactor the code above and merge into something like:
```cpp
if (match(V, m_Sub(m_Value(Op0), m_Value(Op1)))) {
    if (!cast<OverflowingBinaryOperator>(V)->hasNoUnsignedWrap())
      Preconditions.emplace_back(CmpInst::ICMP_ULE, Op1, Op0);
    auto ResA = decompose(Op0, Preconditions, IsSigned, DL);
    auto ResB = decompose(Op1, Preconditions, IsSigned, DL);
    if (!ResA.sub(ResB))
      return ResA;
    return V;
}
```

https://github.com/llvm/llvm-project/pull/209615