reflect: use SelectDir instead of uintptr in runtimeSelect

And fix the wrong comment.

Initially found this because the comment was wrong about the possible
values. Then noticed that there doesn't seem to be any reason to use
uintptr over SelectDir.

Change-Id: I4f9f9640e49d89e558ed00bd99e57dab890785f5
Reviewed-on: https://go-review.googlesource.com/20655Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/value.go

@@ -1876,7 +1876,7 @@ func Copy(dst, src Value) int {
 // A runtimeSelect is a single case passed to rselect.
 // This must match ../runtime/select.go:/runtimeSelect
 type runtimeSelect struct {
-	dir uintptr        // 0, SendDir, or RecvDir
+	dir SelectDir      // SelectSend, SelectRecv or SelectDefault
 	typ *rtype         // channel type
 	ch  unsafe.Pointer // channel
 	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
@@ -1939,7 +1939,7 @@ func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
 	haveDefault := false
 	for i, c := range cases {
 		rc := &runcases[i]
-		rc.dir = uintptr(c.Dir)
+		rc.dir = c.Dir
 		switch c.Dir {
 		default:
 			panic("reflect.Select: invalid Dir")
@@ -2002,7 +2002,7 @@ func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
 	}
 
 	chosen, recvOK = rselect(runcases)
-	if runcases[chosen].dir == uintptr(SelectRecv) {
+	if runcases[chosen].dir == SelectRecv {
 		tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
 		t := tt.elem
 		p := runcases[chosen].val