Auto merge of #130454 - durin42:llvm-20-notrunc, r=workingjubilee

tests: allow trunc/select instructions to be missing

On LLVM 20, these instructions already get eliminated, which at least partially satisfies a TODO. I'm not talented enough at using FileCheck to try and constrain this further, but if we really want to we could copy an LLVM 20 specific version of this test that would restore it to being CHECK-NEXT: insertvalue ...

`@rustbot` label: +llvm-main
r? `@DianQK`

Author: bors