[X86][Costmodel] `getReplicationShuffleCost()`: promote 8 bit-wide elements to 32...

[X86][Costmodel] `getReplicationShuffleCost()`: promote 8 bit-wide elements to 32 bit when no AVX512VBMI

Currently `X86TTIImpl::getInterleavedMemoryOpCostAVX512()` asks about i8 elt type,
so this change does affect vectorization. In the end, it will ask about i1.

We should also try to promote to i16 if we have AVX512BW, i'll do that in a follow-up.
All costs here look good, i've added the missing truncation costs in preparatory patches.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113853