[mlir][spirv] Check nullptr before usage to fix crash

Reviewed By: mravishankar, hanchung

Differential Revision: https://reviews.llvm.org/D123035

Author: antiagainst

mlir/lib/Conversion/ArithmeticToSPIRV/ArithmeticToSPIRV.cpp

@@ -294,10 +294,11 @@ LogicalResult ConstantCompositeOpPattern::matchAndRewrite(
     return failure();
 
   auto dstElementsAttr = constOp.getValue().dyn_cast<DenseElementsAttr>();
-  ShapedType dstAttrType = dstElementsAttr.getType();
   if (!dstElementsAttr)
     return failure();
 
+  ShapedType dstAttrType = dstElementsAttr.getType();
+
   // If the composite type has more than one dimensions, perform linearization.
   if (srcType.getRank() > 1) {
     if (srcType.isa<RankedTensorType>()) {