cmd/compile: emit symbol for constant string before parallel compiler phase
This CL makes sure we walk the newly generated assignment. Part of
that walk makes sure that all symbols for strings are emitted before
we start referencing them during the parallel compilation
phase. Without this change, those references during the parallel phase
do a create-if-not-exist, which leads to a data race.
I'm not 100% sure this is the fix for the issues below, but optimistically
assuming it is...