time: rewrite IsZero method to use wall and ext fields
Using wall and ext fields will be more efficient.
Fixes #76001
Change-Id: If2b9f597562e0d0d3f8ab300556fa559926480a0
GitHub-Last-Rev:
4a91948413079047cb6c382ed29844f456f3064d
GitHub-Pull-Request: golang/go#76006
Reviewed-on: https://go-review.googlesource.com/c/go/+/713720
Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>