[llvm] [DependenceAnalysis] Fix SIV test crash when no AddRec after propagation (PR #154980)

[Ryotaro Kasuga via llvm-commits]

================
@@ -2281,8 +2290,14 @@ bool DependenceInfo::testSIV(const SCEV *Src, const SCEV *Dst, unsigned &Level,
                               Result, NewConstraint) ||
            gcdMIVtest(Src, Dst, Result);
   }
-  llvm_unreachable("SIV test expected at least one AddRec");
-  return false;
+  // If neither expression is an AddRec, this means propagation has simplified
+  // them to non-AddRec forms. In this case, fall back to ZIV analysis since
+  // the expressions are effectively loop-invariant.
+  LLVM_DEBUG(dbgs() << "    falling back to ZIV test due to no AddRec\n");
----------------
kasuga-fj wrote:

The original issue triggers this assertion in the following condition. They are clearly not loop-invariants.
```
SIV
    src = (sext i1 {false,+,true}<%loop> to i64)
    dst = (sext i1 {false,+,true}<%loop> to i64)
SIV test expected at least one AddRec
UNREACHABLE executed at /root/llvm-project/llvm/lib/Analysis/DependenceAnalysis.cpp:2295!
```

https://github.com/llvm/llvm-project/pull/154980