[llvm] [X86] X86FixupVectorConstants - load+sign-extend vector constants that can be stored in a truncated form (PR #79815)

[Phoebe Wang via llvm-commits]

https://github.com/phoebewang approved this pull request.

I have checked the code and a dozen test files. No mistakes found so far, so LGTM.
It's better to pre-commit a NFC patch for refactor change related to rebuildZeroUpperCst and rebuildSplatCst to double check no test affect by potential typos.

https://github.com/llvm/llvm-project/pull/79815