Wrap debug code with the LLVM_DEBUG macro; NFC

While investigating a bug in Clang, I noticed that -Wframe-larger-than
was emitting extra debug information along with the diagnostic. It
turns out that 2e1e2f52 fixed an issue
with the diagnostic, but accidentally left in some debug code that was
exposed in all builds.

So now we no longer emit things like:
8/4294967304 (0.00%) spills, 4294967296/4294967304 (100.00%) variables
along with the diagnostic