runtime: don't clear pointer-free memory when growing maps
runtime: don't clear pointer-free memory when growing maps

If there are no pointers, then clearing memory doesn't help GC,
and the memory is otherwise dead, so don't bother clearing it.

Change-Id: I953f4a3264939f2825e82292030eda2e835cbb97
Reviewed-on: https://go-review.googlesource.com/57350
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Martin Möhrmann <9c82b83f348009b39856043bb9ed3ef41e171aeb@google.com>

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