runtime: avoid division in gc
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 <112830fdd81fb6e3c10661fec6ab883a8a8a909f@intel.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

josharian/go,akutz/go,cloudflare/go,golang/go,codestation/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,ImJasonH/go,ImJasonH/go,vsekhar/elastic-go,ImJasonH/go,CAFxX/go,akutz/go,polyverse-security/go,ImJasonH/go,ImJasonH/go,polyverse-security/go,christopher-henderson/Go,nawawi/go,nawawi/go,codestation/go,polyverse-security/go,golang/go,CAFxX/go,vsekhar/elastic-go,codestation/go,polyverse-security/go,josharian/go,josharian/go,Cofyc/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,ImJasonH/go,cloudflare/go,cloudflare/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,josharian/go,cloudflare/go,josharian/go,codestation/go,vsekhar/elastic-go,cloudflare/go,CAFxX/go,polyverse-security/go,Cofyc/go,Cofyc/go,vsekhar/elastic-go,vsekhar/elastic-go,nawawi/go,akutz/go,nawawi/go,polyverse-security/go,codestation/go,ImJasonH/go,golang/go,ImJasonH/go,Cofyc/go,vsekhar/elastic-go,josharian/go,Cofyc/go,codestation/go,nawawi/go,polyverse-security/go,polyverse-security/go,golang/go,Cofyc/go,codestation/go,CAFxX/go,cloudflare/go,vsekhar/elastic-go,cloudflare/go,golang/go,akutz/go,golang/go,akutz/go,codestation/go,Cofyc/go,akutz/go,golang/go,josharian/go,josharian/go,CAFxX/go,akutz/go