[dev.typeparams] cmd/compile: fix wrong AST generation in devirtualization

CL 330671 moved rewrite method calls to escape analysis. It accidently
made the AST invalid, by removing the OCALLMETH set operation during
devirtualization pass.

Change-Id: I862ffd7f880de55969d7784d9e7b3c38894f6b68
Reviewed-on: https://go-review.googlesource.com/c/go/+/330832


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>