[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)

[Nikita Popov via llvm-commits]

nikic wrote:

> > > Now we are unaware if such a transformation is expected of simplifyLoopIV and if it is also expected to maintain LCSSA.
> > 
> > 
> > simplifyLoopIV is supposed to maintain LCSSA form.
> 
> So, should we update the simplifyLoopIV or just call formLCSSAform after simplifyLoopIV ?

We should fix simplifyLoopIV. Note that it already contains various replacementPreservesLCSSAForm checks already.

https://github.com/llvm/llvm-project/pull/78696