Revert "Refine the expression of valid range"

This reverts commit 4bd1df2.

Author: Mel-Chen

llvm/lib/Analysis/IVDescriptors.cpp

@@ -729,8 +729,8 @@ RecurrenceDescriptor::isFindLastIVPattern(PHINode *OrigPhi, Instruction *I,
     // TODO: This range restriction can be lifted by adding an additional
     // virtual OR reduction.
     const APInt Sentinel = APInt::getSignedMinValue(NumBits);
-    const ConstantRange ValidRange =
-        ConstantRange::getFull(NumBits).difference(ConstantRange(Sentinel));
+    const ConstantRange ValidRange = ConstantRange::getNonEmpty(
+        Sentinel + 1, APInt::getSignedMinValue(NumBits));
     LLVM_DEBUG(dbgs() << "LV: FindLastIV valid range is " << ValidRange
                       << ", and the signed range of " << *AR << " is "
                       << IVRange << "\n");