slices: reduce code nesting depth for Compact and CompactFunc

slices: reduce code nesting depth for Compact and CompactFunc

To make it easier to read.

Change-Id: I2fa1eb78d879b9d86b4dc839be7ede37c7c864f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/581976
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joedian Reid <joedian@google.com>