The chanrecv funcs don't use it at all. The chansend ones do, but the
element type is now part of the hchan struct, which is already a
parameter.

hchan can be nil in chansend when sending to a nil channel, so when
instrumenting we must copy to the stack to be able to read the channel
type.

name             old time/op  new time/op  delta
ChanUncontended  6.42µs ± 1%  6.22µs ± 0%  -3.06%  (p=0.000 n=19+18)

Initially found by github.com/mvdan/unparam.

Fixes #19591.

Change-Id: I3a5e8a0082e8445cc3f0074695e3593fd9c88412
Reviewed-on: https://go-review.googlesource.com/38351
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>