runtime: use wb flag instead of gcphase for allocate-black check
This is an allocator microoptimization. There's no reason to check
gcphase in general, since it's mostly for debugging anyway.
writeBarrier.enabled is set in all the same cases here, and we force one
fewer cache line (probably) to be touched during malloc.
Conceptually, it also makes a bit more sense. The allocate-black policy
is partly informed by the write barrier design.
Change-Id: Ia5ff593d64c29cf7f4d1bced3204056566444a98
Reviewed-on: https://go-review.googlesource.com/c/go/+/617875 Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org>