[SPIRV] Mark maybe unused extractSubvector variable

Change llvm#115178 introduced a variable that is only used in an assert,
which could result in an unused variable warning in builds without
asserts enabled. This just addes the maybe_unused attribute to silence
the warning.

Author: Greg Roth

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

@@ -2941,7 +2941,7 @@ void SPIRVInstructionSelector::extractSubvector(
     Register &ResVReg, const SPIRVType *ResType, Register &ReadReg,
     MachineInstr &InsertionPoint) const {
   SPIRVType *InputType = GR.getResultType(ReadReg);
-  uint64_t InputSize = GR.getScalarOrVectorComponentCount(InputType);
+  [[maybe_unused]] uint64_t InputSize = GR.getScalarOrVectorComponentCount(InputType);
   uint64_t ResultSize = GR.getScalarOrVectorComponentCount(ResType);
   assert(InputSize > 1 && "The input must be a vector.");
   assert(ResultSize > 1 && "The result must be a vector.");