[X86] lowerShuffleAsVTRUNC - support offseted truncations

Extend the <0,Scale,2*Scale,..> pattern to allow for a fixed offset <Offset,Offset+Scale,Offset+2*Scale,..> pattern, which will lower to a single additional bitshift/pshufd.

At the moment I've limited this to cases where the LHS/RHS operands are concatenated for free, but this is only to avoid a couple of regressions that should be easily addressable in followups.