cmd/compile: use ISEL, cleanup use of zero & extensions

Abandoned earlier efforts to expose zero register,
but left it in numbering to decrease squirrelyness of
register allocator.

ISELrelOp used in code generation of bool := x relOp y.
Some patterns added to better elide zero case and
some sign extension.

Updates: #17109

Change-Id: Ida7839f0023ca8f0ffddc0545f0ac269e65b05d9
Reviewed-on: https://go-review.googlesource.com/29380
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -356,6 +356,15 @@ func clobber(v *Value) bool {
 	return true
 }
 
+// noteRule is an easy way to track if a rule is matched when writing
+// new ones.  Make the rule of interest also conditional on
+//     noteRule("note to self: rule of interest matched")
+// and that message will print when the rule matches.
+func noteRule(s string) bool {
+	println(s)
+	return true
+}
+
 // logRule logs the use of the rule s. This will only be enabled if
 // rewrite rules were generated with the -log option, see gen/rulegen.go.
 func logRule(s string) {