Prepare to reenable unreachable-code warnings.

Adding double-parens around an `if ((false))` squelches the warning.
In other cases, you can squelch the warning by assigning the
always-constant(-on-this-machine) check into a constexpr bool.

Change-Id: I5a344fb45779c5bd2865edb3cffaf839ba9a5d85
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/504597
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
46 files changed