[llvm] [LV] Mask all links of a partial-reduction chain when tail-folding. (PR #215875)

[Graham Hunter via llvm-commits]

https://github.com/huntergr-arm approved this pull request.

LGTM.

I wondered about explicitly setting the passthru value of the masked loads to zero instead, but this is a simpler fix and the optimization can be done later.

https://github.com/llvm/llvm-project/pull/215875