[dev.regabi] cmd/compile: untangle FmtFlag, FmtMode

[dev.regabi] cmd/compile: untangle FmtFlag, FmtMode

It turns out that the FmtFlag is really only tracking the FmtLong and FmtShort bits,
and the others simply mirror the state of the FmtMode and are copied out and
back in repeatedly.

Simplify to FmtFlag being the verb itself ('S', 'L', or 'v').

Now there is only one formatting enumeration, making it a bit
easier to understand what's going on.

Passes buildall w/ toolstash -cmp.

Change-Id: I85bde2183eb22228fcf46d19d003401d588d9825
Reviewed-on: https://go-review.googlesource.com/c/go/+/275778
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>