These will appear when tracking live pointers in registers, so we need
to know whether they have pointers.

For #24543.

Change-Id: I2edccee39ca989473db4b3e7875ff166808ac141
Reviewed-on: https://go-review.googlesource.com/108497
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>