cmd/compile: allow pointer-containing elements in stack allocations

cmd/compile: allow pointer-containing elements in stack allocations

For variable-sized allocations.

Turns out that we already implement the correct escape semantics
for this case. Even when the result of the "make" does not escape,
everything assigned into it does.

Change-Id: Ia123c538d39f2f1e1581c24e4135a65af3821c5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/657937
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>