cmd/compile: change Oconv to take an Op parameter

Eliminates type conversions in a bunch of Oconv(int(n.Op), ...) calls.

Notably, this identified a misuse of Oconv in amd64/gsubr.go to try to
print an assembly instruction op instead of a compiler node op.

Change-Id: I93b5aa49fe14a5eaf868b05426d3b8cd8ab52bc5
Reviewed-on: https://go-review.googlesource.com/20298
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/cmd/compile/internal/x86/cgen64.go b/src/cmd/compile/internal/x86/cgen64.go
index 8bb7d37..e59784b 100644
--- a/src/cmd/compile/internal/x86/cgen64.go
+++ b/src/cmd/compile/internal/x86/cgen64.go
@@ -19,12 +19,12 @@
 	if res.Op != gc.OINDREG && res.Op != gc.ONAME {
 		gc.Dump("n", n)
 		gc.Dump("res", res)
-		gc.Fatalf("cgen64 %v of %v", gc.Oconv(int(n.Op), 0), gc.Oconv(int(res.Op), 0))
+		gc.Fatalf("cgen64 %v of %v", gc.Oconv(n.Op, 0), gc.Oconv(res.Op, 0))
 	}
 
 	switch n.Op {
 	default:
-		gc.Fatalf("cgen64 %v", gc.Oconv(int(n.Op), 0))
+		gc.Fatalf("cgen64 %v", gc.Oconv(n.Op, 0))
 
 	case gc.OMINUS:
 		gc.Cgen(n.Left, res)
@@ -531,7 +531,7 @@
 	var br *obj.Prog
 	switch op {
 	default:
-		gc.Fatalf("cmp64 %v %v", gc.Oconv(int(op), 0), t)
+		gc.Fatalf("cmp64 %v %v", gc.Oconv(op, 0), t)
 
 		// cmp hi
 	// jne L