[SLP] Fix issues with debug output in the SLP vectorizer.

The primary issue here is that using NDEBUG alone isn't enough to guard
debug printing -- instead the DEBUG() macro needs to be used so that the
specific pass debug logging check is employed. Without this, every
asserts-enabled build was printing out information when it hit this.

I also fixed another place where we had multiple statements in a DEBUG
macro to use {}s to be a bit cleaner. And I fixed a place that used
`errs()` rather than `dbgs()`.

llvm-svn: 329046