cmd/compile: added some intrinsics to SSA back end

One intrinsic was needed to help get the very best
performance out of a future GC; as long as that one was
being added, I also added Bswap since that is sometimes
a handy thing to have.  I had intended to fill out the
bit-scan intrinsic family, but the mismatch between the
"scan forward" instruction and "count leading zeroes"
was large enough to cause me to leave it out -- it poses
a dilemma that I'd rather dodge right now.

These intrinsics are not exposed for general use.
That's a separate issue requiring an API proposal change
( https://github.com/golang/proposal )

All intrinsics are tested, both that they are substituted
on the appropriate architecture, and that they produce the
expected result.

Change-Id: I5848037cfd97de4f75bdc33bdd89bba00af4a8ee
Reviewed-on: https://go-review.googlesource.com/20564
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -120,6 +120,10 @@ type pass struct {
 // Run consistency checker between each phase
 var checkEnabled = false
 
+// Debug output
+var IntrinsicsDebug int
+var IntrinsicsDisable bool
+
 // PhaseOption sets the specified flag in the specified ssa phase,
 // returning empty string if this was successful or a string explaining
 // the error if it was not.
@@ -157,6 +161,20 @@ func PhaseOption(phase, flag string, val int) string {
 		}
 	}
 
+	if phase == "intrinsics" {
+		switch flag {
+		case "on":
+			IntrinsicsDisable = val == 0
+		case "off":
+			IntrinsicsDisable = val != 0
+		case "debug":
+			IntrinsicsDebug = val
+		default:
+			return fmt.Sprintf("Did not find a flag matching %s in -d=ssa/%s debug option", flag, phase)
+		}
+		return ""
+	}
+
 	underphase := strings.Replace(phase, "_", " ", -1)
 	var re *regexp.Regexp
 	if phase[0] == '~' {