Reduce the minimum benchmark batch size (#6436)

Even with `--benchmark_dry_run`, the benchmarks that use _batching_
still do one batch at a minimum as that's inherent to how batching works
in the benchmark framework.

This means that the minimum batch size can (and in practice does)
trigger timeouts by forcing 1k iterations in the test run that is just
trying to ensure the benchmark doesn't _crash_ in some way.

Reduce the minimum size to 128 instead of 1k for this benchmark which
should put it (much) further from any timeout limit. It also still seems
perfectly effective for getting good benchmark data -- I think the
original value was set _much_ too aggressively.

toolchain/driver/compile_benchmark.cpp

@@ -89,7 +89,7 @@ static auto ComputeFileCount(int target_lines) -> int {
   // Use a smaller number of files in debug builds where compiles are slower.
   return std::max(1, std::min(8, (1024 * 1024) / target_lines));
 #else
-  return std::max(8, std::min(1024, (1024 * 1024) / target_lines));
+  return std::max(8, std::min(128, (1024 * 1024) / target_lines));
 #endif
 }