[X86][AVX] Update VBROADCAST folds to always use v2i64 X86vzload

The VBROADCAST combines and SimplifyDemandedVectorElts improvements mean that we now more consistently use shorter (128-bit) X86vzload input operands.

Follow up to D58053

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354346 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed