[clang] [lld][LTO] Respect `-fno-slp-vectorize` for the LTO pipeline (PR #201585)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Can we use a `cl::opt` option? Can we make `ld.lld -mllvm=-vectorize-slp=0` disable the vectorizer, if it doesn't today?

Done

https://github.com/llvm/llvm-project/pull/201585