Add support for enabling DCHECKs in release mode

Add support for enabling DCHECKs in release mode

When compiling with the macro DCHECK_ALWAYS_ON defined, DCHECKs and
supporting code gets compiled and enabled.

This increases test coverage for chromium release buildbots

Most of this patch was contributed by Ken Russell <kbr@chromium.org>;

BUG=v8:3731
LOG=y
R=jkummerow@chromium.org

[jochen (gone - plz use gerrit), 2014-12-03 09:39:57]

jochen@chromium.org changed reviewers:
+ jkummerow@chromium.org

[jochen (gone - plz use gerrit), 2014-12-03 09:39:58]

ptal

[Jakob Kummerow, 2014-12-03 10:53:53]

LGTM with some comments.

https://codereview.chromium.org/769263002/diff/20001/src/assembler.cc
File src/assembler.cc (right):

https://codereview.chromium.org/769263002/diff/20001/src/assembler.cc#newcode477
src/assembler.cc:477: #if DCHECK_IS_ON
nit: just drop this #if

https://codereview.chromium.org/769263002/diff/20001/src/codegen.h
File src/codegen.h (right):

https://codereview.chromium.org/769263002/diff/20001/src/codegen.h#newcode172
src/codegen.h:172: #ifdef DEBUG
This is gonna cause arm64 compile errors (cf. codegen-arm64.cc).

https://codereview.chromium.org/769263002/diff/20001/src/handles-inl.h
File src/handles-inl.h (right):

https://codereview.chromium.org/769263002/diff/20001/src/handles-inl.h#newcod...
src/handles-inl.h:155: #ifdef DEBUG
Forgot this one? Declaration in handles.h would need updating too.

https://codereview.chromium.org/769263002/diff/20001/src/heap/mark-compact.cc
File src/heap/mark-compact.cc (right):

https://codereview.chromium.org/769263002/diff/20001/src/heap/mark-compact.cc...
src/heap/mark-compact.cc:1301: #if DCHECK_IS_ON
just drop this #if

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc
File src/lithium-allocator.cc (right):

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc...
src/lithium-allocator.cc:68: #ifdef DEBUG
forgotten?

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc...
src/lithium-allocator.cc:81: bool LiveRange::HasOverlap(UseInterval* target)
const {
This is dead code. Delete it while you're here, along with its declaration in
the header file. Please also delete the copy in
compiler/register-allocator.{h,cc}.

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc...
src/lithium-allocator.cc:1333: #ifdef DEBUG
forgotten?

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc...
src/lithium-allocator.cc:2152: #ifdef DEBUG
forgotten?

https://codereview.chromium.org/769263002/diff/20001/src/lithium-allocator.cc...
src/lithium-allocator.cc:2188: #ifdef DEBUG
forgotten?

https://codereview.chromium.org/769263002/diff/20001/src/objects.cc
File src/objects.cc (right):

https://codereview.chromium.org/769263002/diff/20001/src/objects.cc#newcode1071
src/objects.cc:1071: #endif                      // ENABLE_SLOW_DCHECKS
nit: does clang-format stop being stupid when you add an empty line after this?
(again below)