sync: prevent (*Map).Range from always escaping
After the change from CL 426074 the Range method on Map always
escape the read variable, leading to an allocation.
Since the compiler doesn't do live-range splitting for local variables we
need to use some hints to only escape in that particular branch.
Fixes #62404
Change-Id: I938a5e593647455fa827e3dd3ed8ea22c7365df1
GitHub-Last-Rev:
fcbedb467c7b4e6f1d49e299d243cad70deb34e9
GitHub-Pull-Request: golang/go#62408
Reviewed-on: https://go-review.googlesource.com/c/go/+/524976
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>