League-of-legends – Does Irelia’s passive stack with Mercury’s Treads

They stack with each other, as it's not an exclusive buff. Enjoy the extra gold.

Why you shouldn't use Madred's Bloodrazor on Riven:

• Riven scales badly with bonus attack speed, because she should aim to attack only after every ability use. This means not only bonus attack speed from Razor is wasted, but also you won't be using Razor efficiently, since the item procs do scale with attackspeed.
• Riven has no magical attacks, and thus has no need to use Magic Penetration/Reduction. Which means Razor procs will hit enemies full Magic Resistance, making the item even worse.

What to do instead:

• Stack AD. Riven has great AD scaling, and you can always use more of it. Remember that it also makes your shield stronger!
• Stack Lifesteal. Since Riven's passive-boost on autoattacks counts for lifesteal, this can make enormous difference, especially in 1v1. Also, lifesteal scales with the AD you're already stacking. Stacking Bloodthirsters can be legitimate tactic on Riven. If you want advantage earlier in the game, stacking Doran's Blades is also very good.
• Don't forget some Armor Penetration/Reduction. Brutalizer gives good flat bonus for early game, plus some AD and some CDR. For late game, Last Whisper gives percentage-based bonus, and yes - more AD!
• If you are in a teamfight: don't focus the HP stacking opponents - go for the squishies instead. Other people in your team should be better prepared for clearing out the tough guys later (maybe even by having the Bloodrazor, if they can use it better and if the situation warrants it).