runtime: move debug checks behind constant flag in mallocgc
These debug checks are very occasionally helpful, but they do cost real
time. The biggest issue seems to be the bloat of mallocgc due to the
"throw" paths. Overall, after some follow-ups, this change cuts about
1ns off of the mallocgc fast path.
This is a microoptimization that on its own changes very little, but
together with other optimizations and a breaking up of the various
malloc paths will matter all together ("death by a thousand cuts").
Change-Id: I07c4547ad724b9f94281320846677fb558957721
Reviewed-on: https://go-review.googlesource.com/c/go/+/617878
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>