Testcase for a gollvm bug (assert in Llvm_backend::materializeComposite).

Updates golang/go#33020.

Change-Id: Icdf5b4b2b6eb55a5b48a31a61c41215b1ae4cf01
Reviewed-on: https://go-review.googlesource.com/c/go/+/191743Reviewed-by: Ian Lance Taylor <iant@golang.org>