cmd/compile: field tracking is broken again

[dsnet]

Previously broke in #20014

Field tracking is broken again by https://go-review.googlesource.com/40864

The failure can be reproduced by running this test tool: https://godoc.org/rsc.io/tmp/fieldtrack

/cc @josharian

[dsnet]

We really need the builders to test this... :/
\cc @bradfitz

[gopherbot]

CL https://golang.org/cl/43151 mentions this issue.

[dsnet]

@ianlancetaylor and @mdempsky suggested making this a compiler flag so that we dont need to rebuild the entire toolchain just test this. Would be significant step into making this tested by the builders.