[spirv-reader] Fix build

Fix bad merge around emission of OpBitcast.

Change-Id: Iacc5ba1c3940745e4b2c46878bbc416397ec555f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20047
Reviewed-by: David Neto <dneto@google.com>

src/reader/spirv/function.cc

@@ -427,9 +427,8 @@ TypedExpression FunctionEmitter::MaybeEmitCombinatorialValue(
 
   if (inst.opcode() == SpvOpBitcast) {
     auto target_ty = parser_impl_.ConvertType(inst.type_id());
-    auto cast = std::make_unique<ast::AsExpression>(target_ty, operand(0));
-    cast->set_result_type(target_ty);
-    return cast;
+    return {target_ty,
+            std::make_unique<ast::AsExpression>(target_ty, operand(0).expr)};
   }
 
   // builtin readonly function