cmd/compile: enable flag-specified dump of specific phase+function

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>

src/cmd/compile/internal/ssa/func.go

@@ -7,6 +7,7 @@ package ssa
 import (
 	"fmt"
 	"math"
+	"strings"
 )
 
 // A Func represents a Go func declaration (or function literal) and
@@ -113,7 +114,7 @@ func (f *Func) LogStat(key string, args ...interface{}) {
 	}
 	n := "missing_pass"
 	if f.pass != nil {
-		n = f.pass.name
+		n = strings.Replace(f.pass.name, " ", "_", -1)
 	}
 	f.Config.Warnl(f.Entry.Line, "\t%s\t%s%s\t%s", n, key, value, f.Name)
 }