In a function with a `noundef` return value, replace `ret undef` with `unreachable`

[scottmcm]

noundef on the return value means that

If the value representation contains any undefined or poison bits, the behavior is undefined.

so if the function returns undef, that must be unreachable.

Alive2 confirms this is allowed: https://alive2.llvm.org/ce/z/bW2DoM

define i32 @src() noundef {
%start:
  ret i32 undef
}
=>
define i32 @tgt() noundef {
%start:
  assume i1 0
}
Transformation seems to be correct!

But seemingly it doesn't happen today: https://llvm.godbolt.org/z/79a91o59q

[scottmcm]

@dianqk Will https://reviews.llvm.org/D144319 perhaps fix this issue as well?

[dianqk]

@dianqk Will https://reviews.llvm.org/D144319 perhaps fix this issue as well?

Yes, it also solves nonnull related transformations. But this needs to wait until the C++ standard library fixes this UB.
If you need this optimization to be done soon, I can submit a scene that only considers noundef.

@ldionne This UB fix doesn't look like there's been any new progress for about a month now.

[dianqk]

https://reviews.llvm.org/D157227
I added a patch to reduce the impact of this UB.

[dianqk]

Based on the discussion at D157227, we expect to be back in LLVM 18.

cc @nikic @ldionne