Use labelchangemap for PhiNode edges

[tkf]

I just noticed this by reading #41115 and I don't know if there are any problems with this. But it seems to be more appropriate to use labelchangemap for renaming PhiNode edges, considering how other goto labels are renamed? Although I guess we normally have ssachangemap === labelchangemap.

Also, since #41115 is backported, this needs to be backported if merged?

[Keno]

I considered this, but the phi node edges are not "goto" labels, they are "comefrom" labels, so I think the SSA change map for the SSA of the branch node would be the correct map.

[tkf]

Hmm... I thought a change like this would be required if we want to use renumber_ir_elements!((ir::IRCode).stmts.inst, ssachangemap, labelchangemap) where the labels are actually BB indices? But I don't know if there is a use case like this. So, please feel free to close this if you think the code as-is works fine. I just thought it could be a typo or something.

[tkf]

Actually, I just realized that I might have misunderstood the purpose of labelchangemap. I thought it was there for making this generic over the IR formats IRCode/CodeInfo. But it's useful even within CodeInfo where you want to rename SSA uses and labels differently.

[Keno]

renumber_ir_elements!((ir::IRCode).stmts.inst, ssachangemap, labelchangemap)

Consider that GotoNodes need to be changed to the first statements in a BB, while PhiNodes need the last statement in a BB.

[tkf]

Yeah, that's a part of my reasoning just before closing it. For full generality for IRCode, we need three maps ssachangemap, gotolabelchangemap, and philabelchangemap (which I actually needed to implement). "Combining" philabelchangemap into ssachangemap is more useful when the main use is with CodeInfo.