cmd/compile: add countRule rewrite rule helper

noteRule is useful when you're trying to debug
a particular rule, or get a general sense for
how often a rule fires overall.

It is less useful if you're trying to figure
out which functions might be useful to benchmark
to ascertain the impact of a newly added rule.

Enter countRule. You use it like noteRule,
except that you get per-function summaries.

Sample output:

 # runtime
(*mspan).sweep: idx1=1
evacuate_faststr: idx1=1
evacuate_fast32: idx1=1
evacuate: idx1=2
evacuate_fast64: idx1=1
sweepone: idx1=1
purgecachedstats: idx1=1
mProf_Free: idx1=1

This suggests that the map benchmarks
might be good to run for this added rule.

Change-Id: Id471c3231f1736165f2020f6979ff01c29677808
Reviewed-on: https://go-review.googlesource.com/c/go/+/167088
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -5,6 +5,7 @@
 package ssa
 
 import (
+	"bytes"
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"fmt"
@@ -14,6 +15,7 @@ import (
 	"os"
 	"regexp"
 	"runtime"
+	"sort"
 	"strings"
 	"time"
 )
@@ -133,6 +135,21 @@ func Compile(f *Func) {
 		}
 	}
 
+	if f.ruleMatches != nil {
+		var keys []string
+		for key := range f.ruleMatches {
+			keys = append(keys, key)
+		}
+		sort.Strings(keys)
+		buf := new(bytes.Buffer)
+		fmt.Fprintf(buf, "%s: ", f.Name)
+		for _, key := range keys {
+			fmt.Fprintf(buf, "%s=%d ", key, f.ruleMatches[key])
+		}
+		fmt.Fprint(buf, "\n")
+		fmt.Print(buf.String())
+	}
+
 	// Squash error printing defer
 	phaseName = ""
 }