[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:
...
```

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.

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