fix(benchmark): sourcemap benchmark properly test `ConcatSourceMapBui…

…lder` (#4528)

Make 2 changes to sourcemap benchmark:

1. Move counting line breaks in output text to outside of the measured loop. This operation is reasonably expensive, and isn't part of what we're trying to measure.

2. It looks like benchmark is trying to measure concatenating 2 source maps, but `for i in 0..1` was actually only concatenating 1.

tasks/benchmark/benches/sourcemap.rs

@@ -16,15 +16,20 @@ fn bench_sourcemap(criterion: &mut Criterion) {
         group.bench_with_input(id, &file.source_text, |b, source_text| {
             let allocator = Allocator::default();
             let ret = Parser::new(&allocator, source_text, source_type).parse();
+
+            let CodegenReturn { source_text: output_txt, .. } = CodeGenerator::new()
+                .enable_source_map(file.file_name.as_str(), source_text)
+                .build(&ret.program);
+            let lines = output_txt.matches('\n').count() as u32;
+
             b.iter(|| {
-                let CodegenReturn { source_map, source_text } = CodeGenerator::new()
+                let CodegenReturn { source_map, .. } = CodeGenerator::new()
                     .enable_source_map(file.file_name.as_str(), source_text)
                     .build(&ret.program);
-                let line = source_text.matches('\n').count() as u32;
                 if let Some(sourcemap) = source_map {
                     let mut concat_sourcemap_builder = ConcatSourceMapBuilder::default();
-                    for i in 0..1 {
-                        concat_sourcemap_builder.add_sourcemap(&sourcemap, line * i);
+                    for i in 0..2 {
+                        concat_sourcemap_builder.add_sourcemap(&sourcemap, lines * i);
                     }
                     concat_sourcemap_builder.into_sourcemap().to_json_string().unwrap();
                 }