USEFIELD is a special kind of NOP, so treat it like a NOP
when generating the pc-ln table.

There are more invasive fixes that could be applied here.
I am going for minimum number of lines changed.

The smallest test case we know of is five distinct Go files
in four packages, and the bug only happens with
GOEXPERIMENT=fieldtrack enabled, which we don't
normally build with, so the test would never run
meaningfully anyway.

Fixes #5762.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10495044