cmd/compile: normalize spaces in rewrite rule comments.

In addition to look nicer to the eye, this allows to reformat
and indent rules without causing spurious changes to the generated
file, making it easier to spot functional changes.

After this CL, all CLs that will aggregate rules through
the new "|" functionality should cause no changes to the
generated files.

Change-Id: Icec283585ba8d7b91c79d76513c1d83dca4b30aa
Reviewed-on: https://go-review.googlesource.com/95216
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -60,18 +60,22 @@ func (r Rule) String() string {
 	return fmt.Sprintf("rule %q at %s", r.rule, r.loc)
 }
 
+func normalizeSpaces(s string) string {
+	return strings.Join(strings.Fields(strings.TrimSpace(s)), " ")
+}
+
 // parse returns the matching part of the rule, additional conditions, and the result.
 func (r Rule) parse() (match, cond, result string) {
 	s := strings.Split(r.rule, "->")
 	if len(s) != 2 {
 		log.Fatalf("no arrow in %s", r)
 	}
-	match = strings.TrimSpace(s[0])
-	result = strings.TrimSpace(s[1])
+	match = normalizeSpaces(s[0])
+	result = normalizeSpaces(s[1])
 	cond = ""
 	if i := strings.Index(match, "&&"); i >= 0 {
-		cond = strings.TrimSpace(match[i+2:])
-		match = strings.TrimSpace(match[:i])
+		cond = normalizeSpaces(match[i+2:])
+		match = normalizeSpaces(match[:i])
 	}
 	return match, cond, result
 }