Megabus*, runs scheduled service, departing from the RTC South Strip Transfer Terminal (about 3 miles from McCarran airport). One departs LAS at 12:30 AM and arrives in LAX 5:40 AM [overnight, avoiding the excessive heat], even though they are air conditioned.
*Unless these are subcontracted, a "typical" MB is double decker, fairly comfortable seats--a few seats have tables (free WIFI).
PS: I've used MB to go from NYC to Washington, DC (with bookings far enough in advance) I've gone RT for $2.00.