Apply llvm fix for possible hangs with CPUTYPE=skylake-avx512

Merge commit e8305c0b8f49 from llvm git (by Simon Pilgrim)

[X86] combineX86ShuffleChain - don't fold to truncate(concat(V1,V2)) if it was already a PACK op

Fixes #55050

PR: 264394
Reported by: VVD <vvd@unislabs.com>
MFC after: 3 days