[llvm] [RFC][IR] Support vector splats in `ConstantPointerNull` (PR #195486)

[Nikita Popov via llvm-commits]

nikic wrote:

Reduced test case for the instcombine issue:
```
define <4 x i1> @test() {
  %ptr = call ptr @_Znwm(i64 0)
  %gep = getelementptr i8, ptr %ptr, <4 x i64> <i64 4, i64 8, i64 12, i64 16>
  %cmp = icmp eq <4 x ptr> %gep, splat (ptr null)
  ret <4 x i1> %cmp
}

declare ptr @_Znwm(i64)
```

https://github.com/llvm/llvm-project/pull/195486