[llvm] [WIP][X86] lowerBuildVectorAsBroadcast - don't convert constant vectors to broadcasts on AVX512VL targets (PR #73509)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Long term WIP patch while I (slowly) address the various regressions it exposes - don't bother reviewing for now :)

https://github.com/llvm/llvm-project/pull/73509