cmd/compile: reset memcombine correctly between basic blocks

cmd/compile: reset memcombine correctly between basic blocks

Not sure why this bug didn't cause a complete failure, but it
certainly makes for doing a lot more work than is necessary.

Fixes #62604

Change-Id: If0be4acb6eafc3d7eeb42d2f4263c21b4e6e1c7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/527699
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>