Revert "cmd/compile: de-virtualize interface calls"

Revert "cmd/compile: de-virtualize interface calls"

This reverts commit 4e0c7c3f61475116c4ae8d11ef796819d9c404f0.

Reason for revert: The presence-of-optimization test program is fragile, breaks under noopt, and might break if the Go libraries are tweaked.  It needs to be (re)written without reference to other packages.

Change-Id: I3aaf1ab006a1a255f961a978e9c984341740e3c7
Reviewed-on: https://go-review.googlesource.com/38097
Reviewed-by: Keith Randall <khr@golang.org>