Reflect used to communicate to the runtime using interface words,
which is bad for precise GC because sometimes iwords hold a pointer
and sometimes they don't.  This change rewrites channel and select
operations to always pass pointers to the runtime.

reflect.Select gets somewhat more expensive, as we now do an allocation
per receive case instead of one allocation whose size is the max of
all the received types.  This seems unavoidable to get preciseness
(unless we move the allocation into selectgo, which is a much bigger
change).

Fixes #6490

R=golang-codereviews, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/52900043