cmd/compiler/internal/ssagen: refactor code to sort stack vars
Minor refactoring of the code that sorts stack variables to move
from sort.Stable to sort.SliceStable. No change in semantics; this
is intended to lay the groundwork for a future change.
Change-Id: I9eb920e3b3029a734fbe0e0e88c0d57ea3452599
Reviewed-on: https://go-review.googlesource.com/c/go/+/566176
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>