[llvm] [LiveRange] Verify Other LiveRange in Join API (PR #66809)

[via llvm-commits]

annamthomas wrote:

Just for context, this came up because we saw a crash in LiveRange::join() which cannot be reproduced and we do not have debugInfo on the core.

When cross-referencing the asm of the function to the source code, we see the crash is in this code, with S.valno being null:
```
// Rewrite Other values before changing the VNInfo ids.
  // This can leave Other in an invalid state because we're not coalescing
  // touching segments that now have identical values. That's OK since Other is
  // not supposed to be valid after calling join();
  for (Segment &S : Other.segments)
    S.valno = NewVNInfo[RHSValNoAssignments[S.valno->id]];
```
 With Verify() run on`Other` LiveRange, this will be caught. 

https://github.com/llvm/llvm-project/pull/66809