[NFC][LV] Fix warning of unused SubConst variable

llvm#160154 added an assertion
using a new variable, which caused a warning in builds without asserts.
This patch adds [[maybe_unused]] to prevent that warning.

Author: SamTebbs33

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3033,7 +3033,7 @@ class VPExpressionRecipe : public VPSingleDefRecipe {
     assert(Red->getRecurrenceKind() == RecurKind::Add &&
            "Expected an add reduction");
     assert(getNumOperands() >= 3 && "Expected at least three operands");
-    auto *SubConst = dyn_cast<ConstantInt>(getOperand(2)->getLiveInIRValue());
+    [[maybe_unused]] auto *SubConst = dyn_cast<ConstantInt>(getOperand(2)->getLiveInIRValue());
     assert(SubConst && SubConst->getValue() == 0 &&
            Sub->getOpcode() == Instruction::Sub && "Expected a negating sub");
   }