cmd/compile: init info of OAS node in a select case is being dropped [1.16 backport]

[gopherbot]

@rhysh requested issue #48289 to be considered for backport to the next 1.16 minor release.

The problem this fixes was introduced by 0328c3b. It affects the Go 1.17 series, including go1.17.3.

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version go1.16.10 linux/amd64

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version go1.17.3 linux/amd64
FAIL: 1 != 0

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version devel +88e1415d08 Thu Dec 17 04:43:59 2020 +0000 linux/amd64

$ ./bin/go version && ./bin/go run ~/issue48289.go
go version devel +0328c3b660 Thu Dec 17 15:50:38 2020 +0000 linux/amd64
FAIL: 1 != 0

@gopherbot please consider this for backport to 1.17

[rhysh]

gopherbot shouldn't have opened this. The issue doesn't affect the Go 1.16 series, and I tried to follow the instructions on https://github.com/golang/go/wiki/gopherbot#backporting-issues for how to mention the release.

[rhysh]

(I see now: gopherbot looks at the entire comment where I asked for a backport, not only the line where I asked for the backport. And the comment includes the string "1.16". Thanks for closing.)