Move growslice generation to a separate func so that specialization
logic can be shared.
Updates #49480
Change-Id: I9ea5bb898753622d2d767546a46b4db6410dc725
Reviewed-on: https://go-review.googlesource.com/c/go/+/495877 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com>