[X86] LowerVSETCC - always split 512-bit vectors before lowering to PCMPEQ/GT (PR53842)

Extend the existing split where we already do this for v32i16/v64i8

We can end up trying to use PCMPEQ/GT if the result needs to be sign-extended (typically due to the DAGCombiner::foldSextSetcc fold).

Fixes #53842