Avoid unnecessary calls to vm_region_recurse

[fjricci]

Summary

This should significantly improve darwin lsan performance in cases where root regions are not used.

lib/lsan/lsan_common_mac.cc

while (err == KERN_SUCCESS) {
 
     uptr end_address = address + size;
 
     // libxpc stashes some pointers in the Kernel Alloc Once page,
     // make sure not to report those as leaks.
     if (info.user_tag == VM_MEMORY_OS_ALLOC_ONCE) {
       ScanRangeForPointers(address, end_address, frontier, "GLOBAL",
                            kReachable);
+
+      // Recursing over the full memory map is very slow, break out
+      // early if we don't need the full iteration.
+      if (!flags()->use_root_regions || !root_regions->size())
+        break;
     }
 
     // This additional root region scan is required on Darwin in order to
     // detect root regions contained within mmap'd memory regions, because
     // the Darwin implementation of sanitizer_procmaps traverses images
     // as loaded by dyld, and not the complete set of all memory regions.
     //
     // TODO(fjricci) - remove this once sanitizer_procmaps_mac has the same