runtime: avoid division in gc

Replace int division with (cheaper) byte division in heapBitsSetType.
Provides noticeable speed-up:

GrowSlicePtr-6   181ns ± 3%   169ns ± 3%  -6.85%  (p=0.000 n=10+10)

Change-Id: I4064bb72e8e692023783b8f58d19491844c39382
Reviewed-on: https://go-review.googlesource.com/42290
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -1047,7 +1047,9 @@ func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
 					endnb += endnb
 				}
 				// Truncate to a multiple of original ptrmask.
-				endnb = maxBits / nb * nb
+				// Because nb+nb <= maxBits, nb fits in a byte.
+				// Byte division is cheaper than uintptr division.
+				endnb = uintptr(maxBits/byte(nb)) * nb
 				pbits &= 1<<endnb - 1
 				b = pbits
 				nb = endnb