go/ir: do not panic on generic type slice expression
go/ir: do not panic on generic type slice expression

Closes: gh-1270
Closes: gh-1271 [via git-merge-pr]
Co-authored-by: Dominik Honnef <aeacd46bb4fd3f59504c7762d9af6bd04f20aa1f@honnef.co>

dominikh/go-tools,dominikh/go-tools,dominikh/go-tools,dominikh/go-tools