runtime: remove a modulus calculation from pollorder

This is a follow-up to CL 9269, as suggested
by dvyukov.

There is probably even more that can be done
to speed up this shuffle. It will matter more
once CL 7570 (fine-grained locking in select)
is in and can be revisited then, with benchmarks.

Change-Id: Ic13a27d11cedd1e1f007951214b3bb56b1644f02
Reviewed-on: https://go-review.googlesource.com/9393Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/select.go

@@ -243,7 +243,7 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
 	// generate permuted order
 	pollslice := slice{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
 	pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
-	for i := 0; i < int(sel.ncase); i++ {
+	for i := 1; i < int(sel.ncase); i++ {
 		j := int(fastrand1()) % (i + 1)
 		pollorder[i] = pollorder[j]
 		pollorder[j] = uint16(i)