Even inside an "#ifdef DEBUG", we still want to use the DCHECK macro
instead of CHECK in order to get the "correct" error message.

Drive-by: Remove "#ifdef DEBUG" around DCHECKS in macro-assembler-x64.cc

R=ahaas@chromium.org
CC=mtrofin@chromium.org

Change-Id: I5b92c87fa9b10e5751cc2704d6218bee292cfb8f
Reviewed-on: https://chromium-review.googlesource.com/832687Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50184}