cmd/compile: panic on invalid generic append with type parameter spread [1.26 backport]

[gopherbot]

@griesemer requested issue #78163 to be considered for backport to the next 1.26 minor release.

@gopherbot please consider this for backport to 1.26.

[gopherbot]

Change https://go.dev/cl/756124 mentions this issue: [release-branch.go1.26] go/types, types2: handle unconstrained type parameters correctly in a few places

[dmitshur]

Need to confirm whether 1.25 backport is also needed, or if this is 1.26-only per #34536 (comment).

[griesemer]

We haven't really seen issues with this, which is why I think that maybe backport to 1.26 (which is pretty new) may be sufficient.