cmd/compile: remove redundant nil checks

cmd/compile: remove redundant nil checks

Optimize them away if we can.

If not, be more careful about splicing them out after scheduling.

Change-Id: I660e54649d753dc456d2e25d389d375a16d76940
Reviewed-on: https://go-review.googlesource.com/c/go/+/627418
Reviewed-by: Shengwei Zhao <wingrez@126.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>