[llvm] [msan] Fix shadow computation for partially undefined constant fixed-length vectors (PR #143837)

[Thurston Dang via llvm-commits]

thurstond wrote:

> > Given the corollary, do we want to add a flag to revert back to the old behavior? That would make debugging easier.
> 
> If it affects a limited set of tests, I don't think it deserve the flag, otherwise we will have to add them for each patch.

As @fmayer notes, it could be useful for debugging and is fairly easy to implement: https://github.com/llvm/llvm-project/pull/143837/commits/97bfb3444939d7f166fc4b48e7e3fd652ac69496

I'm happy to drop it if you still think it's too much bloat.

https://github.com/llvm/llvm-project/pull/143837