runtime: make mspan.isFree do what's on the tin

Currently mspan.isFree technically returns whether the object was not
allocated *during this cycle*. Fix it so it actually returns whether
or not the object is allocated so the method is more generally useful
(especially for debugging).

It has one caller, which is carefully written to be insensitive to
this distinction, but this lets us simplify this caller.

Change-Id: I9d79cf784a56015e434961733093c1d8d03fc091
Reviewed-on: https://go-review.googlesource.com/30145
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/heapdump.go

@@ -474,7 +474,7 @@ func dumpobjs() {
 			throw("freemark array doesn't have enough entries")
 		}
 
-		for freeIndex := s.freeindex; freeIndex < s.nelems; freeIndex++ {
+		for freeIndex := uintptr(0); freeIndex < s.nelems; freeIndex++ {
 			if s.isFree(freeIndex) {
 				freemark[freeIndex] = true
 			}

src/runtime/mbitmap.go

@@ -264,7 +264,11 @@ func (s *mspan) nextFreeIndex() uintptr {
 	return result
 }
 
+// isFree returns whether the index'th object in s is unallocated.
 func (s *mspan) isFree(index uintptr) bool {
+	if index < s.freeindex {
+		return false
+	}
 	whichByte := index / 8
 	whichBit := index % 8
 	byteVal := *addb(s.allocBits, whichByte)