cmd/internal/gc: fix race build (again)

Add OGETG to the list of ignored operations.

We don't instrument the runtime package, but calls to runtime.getg
can appear in other packages, for example, after inlining
runtime.LockOSThread.

Change-Id: I8d6e91f1f3c8fd1302b596bdead42d588c059911
Reviewed-on: https://go-review.googlesource.com/8553Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/cmd/internal/gc/racewalk.go

@@ -410,9 +410,6 @@ func racewalknode(np **Node, init **NodeList, wr int, skip int) {
 		OLABEL:
 		goto ret
 
-	case OGETG:
-		Yyerror("racewalk: OGETG can happen only in runtime which we don't instrument")
-
 		// does not require instrumentation
 	case OPRINT, // don't bother instrumenting it
 		OPRINTN,     // don't bother instrumenting it
@@ -428,6 +425,11 @@ func racewalknode(np **Node, init **NodeList, wr int, skip int) {
 		ONONAME,
 		OLITERAL,
 		OSLICESTR,
+		// g is goroutine local so cannot race. Although we don't instrument
+		// the runtime package, through inlining the call to runtime.getg can
+		// appear in non runtime packages, for example, after inlining
+		// runtime.LockOSThread.
+		OGETG,
 		// always preceded by bounds checking, avoid double instrumentation.
 		OTYPESW: // ignored by code generation, do not instrument.
 		goto ret