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

[Vedant Paranjape via llvm-commits]

vedantparanjape-amd 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.

I pushed a fix, it adds a replacementPreservesLCSSAForm check to simplifyLoopIV. It passes the testcase as well. @nikic can you please check once, thanks !

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