cmd/compile: TestFormats failing on the longtest builder

[ALTree]

Since CL 146957, the TestFormat test in cmd/compile is failing, since the list of the known formats is out-of-date.

Longtest builder failure:
https://build.golang.org/log/be3027b68b48c2a170b6bcf059923fc841383eb3

Fix incoming.

[gopherbot]

Change https://golang.org/cl/147837 mentions this issue: cmd/compile: add new format to known_formats

[gopherbot]

Change https://golang.org/cl/205440 mentions this issue: cmd/compile: do not skip TestFormats on short builders