[dev.ssa] cmd/compile: Don't rematerialize getg

It isn't safe in functions that also call setg.

Change-Id: I76a7bf0401b4b6c8a129c245b15a2d6f06080e94
Reviewed-on: https://go-review.googlesource.com/16095Reviewed-by: Todd Neal <todd@tneal.org>

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

@@ -982,7 +982,11 @@ func (v *Value) rematerializeable() bool {
 		// which can't be moved.
 		return false
 	}
-	// TODO: maybe not OpAMD64LoweredGetG?
+	if v.Op == OpAMD64LoweredGetG {
+		// It would almost always be ok to rematerialize this op.
+		// The annoying exception is functions that call runtime.setg.
+		return false
+	}
 	if len(v.Args) == 0 {
 		return true
 	}