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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@329046 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm