[llvm] [LV] Don't vectorize if trip count expansion may introduce UB. (PR #92177)

[Eli Friedman via llvm-commits]

================
@@ -161,6 +161,11 @@ SCEVExpander::findInsertPointAfter(Instruction *I,
   return IP;
 }
 
+bool SCEVExpander::expansionMayIntroduceUB(const SCEV *Expr) {
+  return SCEVExprContains(Expr,
+                          [](const SCEV *Op) { return isa<SCEVUDivExpr>(Op); });
----------------
efriedma-quic wrote:

Is this the same as SafeToHoist in SCEVExpander::expand?

We could probably do something more clever if it mattered: the only way for the exit count to contain UB is if the branch never executes, so it would be correct to just transform `a/b` to `a/umax(b, 1)`.

https://github.com/llvm/llvm-project/pull/92177