runtime: make garbage collector faster by deleting code again

Remove GC bitmap backward scanning.
This was already done once in https://golang.org/cl/5530074/
Still makes GC a bit faster.
On the garbage benchmark, before:
        gc-pause-one=237345195
        gc-pause-total=4746903
        cputime=32427775
        time=32458208
after:
        gc-pause-one=235484019
        gc-pause-total=4709680
        cputime=31861965
        time=31877772
Also prepares mgc0.c for future changes.

R=golang-codereviews, khr, khr
CC=golang-codereviews, rsc
https://golang.org/cl/105380043

src/pkg/runtime/mgc0.c

@@ -270,7 +270,7 @@ static bool
 markonly(void *obj)
 {
 	byte *p;
-	uintptr *bitp, bits, shift, x, xbits, off, j;
+	uintptr *bitp, bits, shift, x, xbits, off;
 	MSpan *s;
 	PageID k;
 
@@ -298,18 +298,6 @@ markonly(void *obj)
 		goto found;
 	}
 
-	// Pointing just past the beginning?
-	// Scan backward a little to find a block boundary.
-	for(j=shift; j-->0; ) {
-		if(((xbits>>j) & (bitAllocated|bitBlockBoundary)) != 0) {
-			shift = j;
-			bits = xbits>>shift;
-			if(CollectStats)
-				runtime·xadd64(&gcstats.markonly.foundword, 1);
-			goto found;
-		}
-	}
-
 	// Otherwise consult span table to find beginning.
 	// (Manually inlined copy of MHeap_LookupMaybe.)
 	k = (uintptr)obj>>PageShift;
@@ -424,7 +412,7 @@ static void
 flushptrbuf(Scanbuf *sbuf)
 {
 	byte *p, *arena_start, *obj;
-	uintptr size, *bitp, bits, shift, j, x, xbits, off, nobj, ti, n;
+	uintptr size, *bitp, bits, shift, x, xbits, off, nobj, ti, n;
 	MSpan *s;
 	PageID k;
 	Obj *wp;
@@ -496,19 +484,6 @@ flushptrbuf(Scanbuf *sbuf)
 
 		ti = 0;
 
-		// Pointing just past the beginning?
-		// Scan backward a little to find a block boundary.
-		for(j=shift; j-->0; ) {
-			if(((xbits>>j) & (bitAllocated|bitBlockBoundary)) != 0) {
-				obj = (byte*)obj - (shift-j)*PtrSize;
-				shift = j;
-				bits = xbits>>shift;
-				if(CollectStats)
-					runtime·xadd64(&gcstats.flushptrbuf.foundword, 1);
-				goto found;
-			}
-		}
-
 		// Otherwise consult span table to find beginning.
 		// (Manually inlined copy of MHeap_LookupMaybe.)
 		k = (uintptr)obj>>PageShift;