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/gc/main.go

@@ -246,6 +246,7 @@ func Main() {
 				continue
 			}
 			val := 1
+			valstring := ""
 			if i := strings.Index(name, "="); i >= 0 {
 				var err error
 				val, err = strconv.Atoi(name[i+1:])
@@ -253,6 +254,9 @@ func Main() {
 					log.Fatalf("invalid debug value %v", name)
 				}
 				name = name[:i]
+			} else if i := strings.Index(name, ":"); i >= 0 {
+				valstring = name[i+1:]
+				name = name[:i]
 			}
 			for _, t := range debugtab {
 				if t.name == name {
@@ -273,7 +277,7 @@ func Main() {
 					flag = phase[i+1:]
 					phase = phase[:i]
 				}
-				err := ssa.PhaseOption(phase, flag, val)
+				err := ssa.PhaseOption(phase, flag, val, valstring)
 				if err != "" {
 					log.Fatalf(err)
 				}