runtime: node ordering in mTreap; adjust code to reflect description.

Adjust mTreap ordering logic to reflect the description of mTreap ordering.
Before it was using unsafe.Pointer in order to gather the base address of
a span. This has been changed to use base, which is the startAddress of a
span as the description is telling us in mgclarge.go.

Fixes: golang/go#28805

Change-Id: Ib3cd94a0757e23d135b5d41830f38fc08bcf16a3
GitHub-Last-Rev: 93f749b6700b1e179de16607a18395d5e162ecc1
GitHub-Pull-Request: golang/go#28973
Reviewed-on: https://go-review.googlesource.com/c/151499Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

src/runtime/mgclarge.go

@@ -164,10 +164,10 @@ func (root *mTreap) insert(span *mspan) {
 			pt = &t.right
 		} else if t.npagesKey > npages {
 			pt = &t.left
-		} else if uintptr(unsafe.Pointer(t.spanKey)) < uintptr(unsafe.Pointer(span)) {
+		} else if t.spanKey.base() < span.base() {
 			// t.npagesKey == npages, so sort on span addresses.
 			pt = &t.right
-		} else if uintptr(unsafe.Pointer(t.spanKey)) > uintptr(unsafe.Pointer(span)) {
+		} else if t.spanKey.base() > span.base() {
 			pt = &t.left
 		} else {
 			throw("inserting span already in treap")
@@ -271,9 +271,9 @@ func (root *mTreap) removeSpan(span *mspan) {
 			t = t.right
 		} else if t.npagesKey > npages {
 			t = t.left
-		} else if uintptr(unsafe.Pointer(t.spanKey)) < uintptr(unsafe.Pointer(span)) {
+		} else if t.spanKey.base() < span.base() {
 			t = t.right
-		} else if uintptr(unsafe.Pointer(t.spanKey)) > uintptr(unsafe.Pointer(span)) {
+		} else if t.spanKey.base() > span.base() {
 			t = t.left
 		}
 	}