-M, -P, and -R were for debugging backend passes that no longer
exists.

-g is used for debugging instructions generated with Gins, but the SSA
backend mostly generates instructions directly. The handful of
instructions still generated with Gins are pretty useless for
debugging.

-x was used to debug the old lexer, but now it only causes us to print
file names as they're parsed, and only if we manually hack the
compiler to enable tracing.

Change-Id: Ia58d4bc9c1312693466171a3fcefc1221e9a2381
Reviewed-on: https://go-review.googlesource.com/32428
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>