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

[Vitaly Buka via llvm-commits]

vitalybuka 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.

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