Gosh, it would have been interesting to see how this played out! Now I really wish the truth had come out -- maybe from Xander? - while Spike was on Angel.
I think Angel would have wanted Spike dead. I think he would have been as enraged as everyone here said. I think he would have tried to kill Spike.
Now, whether or not I think he would have succeeded is up for debate. First off, certainty and righteous indignation are powerful things. Angel's emotions might have given him the upper hand, especially if he told Spike why he suddenly wanted to go ten rounds with him. If Spike knew why, I don't think he would fight back as hard. That thought always kind of paralyzed him. After all, that's the thing that made him go and win his soul. On the other hand, he might fight back even harder and tell Angel that if anyone's gonna take him out for that, it's gonna be Buffy. And we all know Spike can beat Angel in a fight.
Then, we have to consider that Angel already knew Buffy and Spike had a sexual relationship. It's not a stretch to say he might have guessed something to the effect of the violent way it all played out. He smelled Spike on her when he showed up on Buffy for the cookies conversation. He was surprised and hurt and angry even then, but in an amazing leap, he actually shut up about it and lived with a Buffy decision that he didn't like when Buffy said "he's... in my heart" and acted like she didn't want to discuss Spike. It comes up later in the Angel/Spike relationship when Spike's ghost first comes back and Angel clarifies the situation for the team, in Destiny when they fight for the cup, and in The Girl in Question when they make asses out of themselves all over Italy. I can think of at least twice when Spike tells angel that he had an ongoing sexual relationship with Buffy: in Destiny, he says something like (all those times I was puttin' it to her;' and in The Girl in Question, when Angel says sex doesn't make something a relationship, Spike's retort is, "It does if you do it enough." Oh - and when Angel was delusional from that parasite thing, (and Spike saved him - bahaha) he had a nightmare about Spike having sex with Buffy. Angel knew about that and didn't try to kill Spike. He even helped Spike (like with the hands thing). So he can't be mate guarding too much, here. Given how well he knows them and that he knows the timeline, I think might not be surprised when he hears about how ugly things eventually got between Buffy and Spike. He might have figured it out on his own and, so far, he has not tried to kill Spike. I think the soul means a lot to Angel.
Then again, in Angel's nightmare about Spike sleeping with Buffy, he imagined something a heck of a lot more tame than anything we ever saw between them. That's more like what we saw with Angel and Buffy (that's not a slight - he should have been gentle, given that it was Buffy's first time), but nothing like what we saw with Spike and, well, anyone. So maybe Angel has no idea.
Anyway, I'm torn on the issue. I do hope Joss lets us know in the comics! If nothing else, they'd have a pretty darn interesting conversation.