cmd/compile: struct interface{} value lost passing by value [1.12 backport]

[gopherbot]

@randall77 requested issue #30956 to be considered for backport to the next 1.12 minor release.

@gopherbot, please consider this for backport to 1.12, it's a regression.

[gopherbot]

Change https://golang.org/cl/170437 mentions this issue: [release-branch.go1.12] cmd/compile: fix literal struct interface {} lost passing by value

[gopherbot]

Closed by merging 9915830 to release-branch.go1.12.