cmd/compile: fix interaction between GOEXPERIMENT=fieldtrack and race detector

Tested by hand.
Only lines of code changing are protected by Fieldtrack_enabled > 0,
which is never true in standard Go distributions.

Fixes #12171.

Change-Id: I963b9997dac10829db8ad4bfc97a7d6bf14b55c6
Reviewed-on: https://go-review.googlesource.com/13676


Reviewed-by: Ian Lance Taylor <iant@golang.org>