[clang] [flang] [flang] Add -f[no-]slp-vectorize flags (PR #132801)

[Tarun Prabhu via cfe-commits]

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

Apart from the docstrings, this looks good. Thanks for the changes :-)

https://github.com/llvm/llvm-project/pull/132801