Revert "cmd/compile/internal: merge stack slots for selected local auto vars"

Revert "cmd/compile/internal: merge stack slots for selected local auto vars"

This reverts CL 553055.

Reason for revert: causes crypto/ecdsa failures on linux ppc64/s390x builders

Change-Id: I9266b030693a5b6b1e667a009de89d613755b048
Reviewed-on: https://go-review.googlesource.com/c/go/+/575236
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>