swift234

Disable the LLVM SLP vectorizer for the stdlib build.

This shaves around 5 seconds off the stdlib build if you're using a
release/no-assert LLVM (so around 12.5% of total time if you're also
using a release/no-assert swift, or around 4% of total time if you're
using a debug/assert swift).

The underlying LLVM issue is tracked by rdar://problem/19430955.

I saw no significant reproducible performance loss due to this change.

Resolves rdar://problem/19431821.

Swift SVN r24342