runtime: race instrument read of convT2E/I arg

Sometimes this read is instrumented by compiler when it creates
a temp to take address, but sometimes it is not (e.g. for global vars
compiler takes address of the global directly).

Instrument convT2E/I similarly to chansend and mapaccess.

Fixes #12664

Change-Id: Ia7807f15d735483996426c5f3aed60a33b279579
Reviewed-on: https://go-review.googlesource.com/14752


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>