[llvm] [SLP]Support for zext i1 %x modeling as select %x, 1, 0 (PR #180635)

[Devon Loehr via llvm-commits]

DKLoehr wrote:

@Andarwinux: Thanks for reporting this and especially providing a repro file! I'm here to report a similar-looking crash when compiling chromium, it would be good to confirm they're the same:
```
Assertion failed: (detail::isPresent(Val) && "dyn_cast on a non-existent value"), function dyn_cast, file Casting.h, line 656.
```

Lines (3) and (4) look the same as the above snippet so I suspect it's the same issue.

https://github.com/llvm/llvm-project/pull/180635