[X86] Split multi-line chained assignments into single lines to avoid making clang...

[X86] Split multi-line chained assignments into single lines to avoid making clang-format create triangle shaped indentation. Simplify one if statement to remove a bunch of string matches. NFCI

We had an if statement that checked over every avx512* feature to see if it should enabled avx512f. Since they are all prefixed with avx512 just check for that instead.

llvm-svn: 361557