cmd/compile: aggregate scalar allocations for heap escapes
If multiple small scalars escape to the heap, allocate them together
with a single allocation. They are going to be aggregated together
in the tiny allocator anyway, might as well do just one runtime call.
Change-Id: I4317e29235af63de378a26436a18d7fb0c39e41f
Reviewed-on: https://go-review.googlesource.com/c/go/+/648536 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>