…in dead store elimination

Update #63743

Change-Id: I163c6038c13d974dc0ca9f02144472bc05331826
Reviewed-on: https://go-review.googlesource.com/c/go/+/538595
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
(cherry picked from commit 43b57b8)
Reviewed-on: https://go-review.googlesource.com/c/go/+/538857
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

…s after the nil check

Have nil checks return a pointer that is known non-nil. Users of
that pointer can use the result, ensuring that they are ordered
after the nil check itself.

The order dependence goes away after scheduling, when we've fixed
an order. At that point we move uses back to the original pointer
so it doesn't change regalloc any.

This prevents pointer arithmetic on nil from being spilled to the
stack and then observed by a stack scan.

Fixes #63743

Change-Id: I1a5fa4f2e6d9000d672792b4f90dfc1b7b67f6ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/537775
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
(cherry picked from commit 962ccbe)
Reviewed-on: https://go-review.googlesource.com/c/go/+/538717
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>