For very large input files, use of GOSSAFUNC to obtain a dump
after compilation steps can lead to both unwieldy large output
files and unwieldy larger processes (because the output is
buffered in a string).  This flag

  -d=ssa/<phase>/dump:<function name>

provides finer control of what is dumped, into a smaller
file, and with less memory overhead in the running compiler.
The special phase name "build" is added to allow printing
of the just-built ssa before any transformations are applied.

This was helpful in making sense of the gogo/protobuf
problems.

The output format was tweaked to remove gratuitous spaces,
and a crude -d=ssa/help help text was added.

Change-Id: If7516e22203420eb6ed3614f7cee44cb9260f43e
Reviewed-on: https://go-review.googlesource.com/23044
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>