cmd/compile: handle loops better during stack allocation of slices
Don't use the move2heap optimization if the move2heap is inside
a loop deeper than the declaration of the slice. We really only want
to do the move2heap operation once.
Change-Id: I4a68d01609c2c9d4e0abe4580839e70059393a81
Reviewed-on: https://go-review.googlesource.com/c/go/+/722440 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>