cmd/compile: silence esc diagnostics about directiface OCONVIFACEs

In general, a conversion to interface type may require values to be
boxed, which in turn necessitates escape analysis to determine whether
the boxed representation can be stack allocated.

However, esc.go used to unconditionally print escape analysis
decisions about OCONVIFACE, even for conversions that don't require
boxing (e.g., pointers, channels, maps, functions).

For test compatibility with esc.go, escape.go similarly printed these
useless diagnostics. This CL removes the diagnostics, and updates test
expectations accordingly.

Change-Id: I97c57a4a08e44d265bba516c78426ff4f2bf1e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/192697


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>