[llvm] [SelectionDAG] Fix soft-promotion of f16 BUILD_VECTOR for MSA targets (PR #210515)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@fo40225 avoid using rebase/force push once a PR is under review, especially for small changes like I suggested, it can destroy the review history

https://github.com/llvm/llvm-project/pull/210515