[llvm] [SCCP] Handle llvm.vscale intrinsic calls (PR #114033)

[Hari Limaye via llvm-commits]

hazzlim wrote:

> > > I'm not opposed to handling more intrinsics in SCCP. But do we really need it? Can you provide some tests that cannot be simplified by InstSimplify/InstCombine?
> > 
> > 
> > The motivating case for this is actually to make use of the constantrange information in Function Specialization. We see cases like the following:
> > ```
> > define i32 @void(i32 %x) vscale_range(1, 16) {
> > ...
> >   %scale = call i32 @llvm.vscale.i32
> >   %bound = shl nsw nuw i32 %scale, 3
> >   %cmp = icmp ult i32 %x, %bound
> >   br i1 %cmp, label %after, label %work
> > 
> > work:
> > ...do a lot of work...
> >   br label %after
> > 
> > after:
> > ...
> > ```
> > 
> > 
> >     
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> >   
> > We currently aren't able to detect that a specialization candidate, e.g. `{ x = 1 }`, can make the `%work` block dead, because in IPSCCP the SCCPSolver hasn't found `%bound` to have a constant range. This prevents us from properly estimating the codesize savings for a specialization.
> 
> Make sense to me. Can you add this case?

I've added a similar test case of branch elimination in SCCP. The full case in FunctionSpecialization case requires an additional change (in https://github.com/llvm/llvm-project/pull/114073)

https://github.com/llvm/llvm-project/pull/114033