cmd/compile: gracefully fail when devirtualization fails
We should still be able to devirtualize here, but I need to understand
the AST better. While I'm doing that, at least switch to a graceful
failure case (i.e., skip the optimization and print a warning message)
to fix the x/text builders.
Updates #42279.
Change-Id: Ie2b0b701fccf590d0cabfead703fc2fa999072cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/266359
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>