devel/llvm12: fix 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: 264480
MFH: 2022Q2
Sponsored by: DARPA, AFRL