cmd/compile: don't crash on -d=ssa/

I forgot how to pull up the ssa debug options help, so instead of
writing -d=ssa/help, I just wrote -d=ssa/. Much to my amusement, the
compiler just crashed, as shown below. Fix that.

	panic: runtime error: index out of range

	goroutine 1 [running]:
	cmd/compile/internal/ssa.PhaseOption(0x7ffc375d2b70, 0x0, 0xdbff91, 0x5, 0x1, 0x0, 0x0, 0x1, 0x1)
	    /home/mvdan/tip/src/cmd/compile/internal/ssa/compile.go:327 +0x1876
	cmd/compile/internal/gc.Main(0xde7bd8)
	    /home/mvdan/tip/src/cmd/compile/internal/gc/main.go:411 +0x41d0
	main.main()
	    /home/mvdan/tip/src/cmd/compile/main.go:51 +0xab

Change-Id: Ia2ad394382ddf8f4498b16b5cfb49be0317fc1aa
Reviewed-on: https://go-review.googlesource.com/c/154421
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -212,7 +212,8 @@ var BuildDump string // name of function to dump after initial build of ssa
 // BOOT_GO_GCFLAGS=-d='ssa/~^.*scc$/off' GO_GCFLAGS='-d=ssa/~^.*scc$/off' ./make.bash
 //
 func PhaseOption(phase, flag string, val int, valString string) string {
-	if phase == "help" {
+	switch phase {
+	case "", "help":
 		lastcr := 0
 		phasenames := "    check, all, build, intrinsics"
 		for _, p := range passes {

src/cmd/go/testdata/script/gcflags_patterns.txt

@@ -27,6 +27,10 @@ stderr 'compile.* -e.* -p z1'
 go test -c -n -gcflags='all=-e' z1
 stderr 'compile.* -e.* -p z3 '
 
+# this particular -gcflags argument made the compiler crash
+! go build -gcflags=-d=ssa/ z1
+stderr 'PhaseOptions usage'
+
 # -ldflags for implicit test package applies to test binary
 go test -c -n -gcflags=-N -ldflags=-X=x.y=z z1
 stderr 'compile.* -N .*z_test.go'