runtime: avoid conditional execution in morePointers and isPointer

heapBits.bits is carefully written to produce good machine code. Use
it in heapBits.morePointers and heapBits.isPointer to get good machine
code there, too.

Change-Id: I208c7d0d38697e7a22cad67f692162589b75f1e2
Reviewed-on: https://go-review.googlesource.com/22630Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mbitmap.go

@@ -492,7 +492,7 @@ func (h heapBits) bits() uint32 {
 // are scalars.
 // h must not describe the first or second word of the object.
 func (h heapBits) morePointers() bool {
-	return *h.bitp&(bitMarked<<h.shift) != 0
+	return h.bits()&bitMarked != 0
 }
 
 // isPointer reports whether the heap bits describe a pointer word.
@@ -501,7 +501,7 @@ func (h heapBits) morePointers() bool {
 // nosplit because it is used during write barriers and must not be preempted.
 //go:nosplit
 func (h heapBits) isPointer() bool {
-	return (*h.bitp>>h.shift)&bitPointer != 0
+	return h.bits()&bitPointer != 0
 }
 
 // hasPointers reports whether the given object has any pointers.