cmd/compile: tweak offset-generator to elide more +0 offsets

cmd/compile: tweak offset-generator to elide more +0 offsets

this caused a problem in write barrier code when a spurious
zero-offset prevented a write barrier elision.

removed cache after instrumenting it and discovering
zero safe hits (one value must dominate the other, else
unsafe).

Change-Id: I42dfdb4d38ebfe158b13e766a7fabfc514d773f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/297349
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>