perf block-range: Move debug code behind ifndef NDEBUG

perf block-range: Move debug code behind ifndef NDEBUG

Make good on a comment and avoid a unused-but-set-variable warning.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230330183827.1412303-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>