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/gc/go.go

@@ -336,6 +336,9 @@ const (
 
 	// Instruction updates whichever of from/to is type D_OREG. (ppc64)
 	PostInc = 1 << 29
+
+	// Optional 3rd input operand, only ever read.
+	From3Read = 1 << 30
 )
 
 type Arch struct {