cmd/compile: unreachable panic with GODEBUG=gotypesalias=1 [1.22 backport]

[gopherbot]

@findleyr requested issue #65778 to be considered for backport to the next 1.22 minor release.

@gopherbot please consider this for backport to 1.22. It is a panic that prevents tool developers from preparing their tool for go/types Aliases.

[gopherbot]

Change https://go.dev/cl/565840 mentions this issue: [release-branch.go1.22] go/types, types2: handle Alias types in substitution

[mknyszek]

Approved because it's a serious issue affecting tool developers. It's also safe because it's behind a GODEBUG flag, so most users won't run into any new issues introduced.

[gopherbot]

Closed by merging e23707b to release-branch.go1.22.