[llvm] [InstCombine] Try the flipped strictness of predicate in `foldICmpShlConstant` (PR #92773)

[Nikita Popov via llvm-commits]

================
@@ -2414,14 +2414,34 @@ Instruction *InstCombinerImpl::foldICmpShlConstant(ICmpInst &Cmp,
   // free on the target. It has the additional benefit of comparing to a
   // smaller constant that may be more target-friendly.
   unsigned Amt = ShiftAmt->getLimitedValue(TypeBits - 1);
-  if (Shl->hasOneUse() && Amt != 0 && C.countr_zero() >= Amt &&
-      DL.isLegalInteger(TypeBits - Amt)) {
-    Type *TruncTy = IntegerType::get(Cmp.getContext(), TypeBits - Amt);
-    if (auto *ShVTy = dyn_cast<VectorType>(ShType))
-      TruncTy = VectorType::get(TruncTy, ShVTy->getElementCount());
-    Constant *NewC =
-        ConstantInt::get(TruncTy, C.ashr(*ShiftAmt).trunc(TypeBits - Amt));
-    return new ICmpInst(Pred, Builder.CreateTrunc(X, TruncTy), NewC);
+  if (Shl->hasOneUse() && Amt != 0 &&
+      shouldChangeType(ShType->getScalarSizeInBits(), TypeBits - Amt)) {
+    ICmpInst::Predicate CmpPred = Pred;
+    APInt RHSC = C;
+
+    if (RHSC.countr_zero() < Amt && ICmpInst::isStrictPredicate(CmpPred)) {
+      // Try the flipped strictness predicate.
+      // e.g.:
+      // icmp ult i64 (shl X, 32), 8589934593 ->
+      // icmp ule i64 (shl X, 32), 8589934592 ->
+      // icmp ule i32 (trunc X, i32), 2 ->
+      // icmp ult i32 (trunc X, i32), 3
----------------
nikic wrote:

```suggestion
      // Try the flipped strictness predicate. For example:
      // icmp ult i64 (shl X, 32), 8589934593 ->
      // icmp ule i64 (shl X, 32), 8589934592 ->
```
The second example is not applicable due to the isStrictPredicate check.

https://github.com/llvm/llvm-project/pull/92773