Use FlagValue::value() in SLOW_DCHECK

Previously SLOW_DCHECK used the non-constexpr bool() operator
of FlagValue, which cannot be used in constexpr. Switch to
FlagValue::value() instead for make it compile in constexpr.

Change-Id: I3e4f70d82c0027cf56999b6c4639479606151696
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4341495
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/main@{#86611}

src/common/checks.h

@@ -15,7 +15,7 @@
 
 #ifdef ENABLE_SLOW_DCHECKS
 #define SLOW_DCHECK(condition) \
-  CHECK(!v8::internal::v8_flags.enable_slow_asserts || (condition))
+  CHECK(!v8::internal::v8_flags.enable_slow_asserts.value() || (condition))
 #define SLOW_DCHECK_IMPLIES(lhs, rhs) SLOW_DCHECK(!(lhs) || (rhs))
 #else
 #define SLOW_DCHECK(condition) ((void)0)