[ValueTracking][RISCV] Use ConstantRange::getUnsignedMax instead of getUpper to simplify some code. (#96816)

This avoids the need to subtract 1 and explain why.

Author: topperc

llvm/lib/Analysis/ValueTracking.cpp

@@ -1740,10 +1740,8 @@ static void computeKnownBitsFromOperator(const Operator *I,
             cast<ConstantInt>(II->getArgOperand(HasAVL))->getZExtValue());
         RISCVII::VLMUL VLMUL = static_cast<RISCVII::VLMUL>(
             cast<ConstantInt>(II->getArgOperand(1 + HasAVL))->getZExtValue());
-        // The Range is [Lower, Upper), so we need to subtract 1 here to get the
-        // real upper value.
         uint64_t MaxVLEN =
-            (Range.getUpper().getZExtValue() - 1) * RISCV::RVVBitsPerBlock;
+            Range.getUnsignedMax().getZExtValue() * RISCV::RVVBitsPerBlock;
         uint64_t MaxVL = MaxVLEN / RISCVVType::getSEWLMULRatio(SEW, VLMUL);
 
         // Result of vsetvli must be not larger than AVL.