cmd/compile: fix failure to reset reused bit of storage

This is the "3rd bug" that caused compilations to sometimes
produce different results when dwarf location lists were
enabled.

A loop had not been properly rewritten in an earlier
optimization CL, and it accessed uninitialized data,
which was deterministically perhaps wrong when single
threaded, but variably wrong when multithreaded.

Change-Id: Ib3da538762fdf7d5e4407106f2434f3b14a1d7ea
Reviewed-on: https://go-review.googlesource.com/99935


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>