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

[Florian Mayer via llvm-commits]

fmayer wrote:

> otherwise we will have to add them for each patch.

For this patch it is just extremely easy because it's all contained anyway. But no super strong opinion.

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