Improve asserts

Author: mcourteaux

src/CodeGen_GPU_Dev.cpp

@@ -161,9 +161,7 @@ void CodeGen_GPU_C::visit(const Shuffle *op) {
 
         std::vector<std::string> vecs;
         for (const Expr &v : op->vectors) {
-            std::string expr = print_expr(v);
-            internal_assert(!expr.empty()) << "Expression did not serialize.";
-            vecs.push_back(std::move(expr));
+            vecs.push_back(print_expr(v));
         }
 
         std::string src = vecs[0];
@@ -190,8 +188,8 @@ void CodeGen_GPU_C::visit(const Shuffle *op) {
                     break;
                 }
             }
-            internal_assert(lane_idx != -1) << "Shuffle lane index not found.";
-            internal_assert(vector_idx < op->vectors.size()) << "Shuffle vector index not found.";
+            internal_assert(lane_idx != -1) << "Shuffle lane index not found: i=" << i;
+            internal_assert(vector_idx < op->vectors.size()) << "Shuffle vector index not found: i=" << i << ", lane=" << lane_idx;
             rhs << vecs[vector_idx];
             if (op->vectors[vector_idx].type().lanes() > 1) {
                 switch (vector_declaration_style) {