Currently, when the mutator allocates, the runtime first allocates the
memory and then, if that G has done "enough" allocation, the runtime
checks whether the G has assist debt to pay off and, if so, pays it
off. This approach leads to under-assisting, where a G can allocate a
large region (or many small regions) before paying for it, or can even
exit with outstanding debt.

This commit flips this around so that a G always acquires enough
credit for an allocation before it can perform that allocation. We
continue to amortize the cost of assists by requiring that they
over-assist when triggered to build up credit for many allocations.

Fixes #11967.

Change-Id: Idac9f11133b328535667674d837be72c23ebd899
Reviewed-on: https://go-review.googlesource.com/15409Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>