[X86] combineEXTRACT_SUBVECTOR - extract 256-bit comparisons if only one subvector is required

If only one subvector extraction will be necessary (i.e. because the other is constant etc.) then extract the source operands and perform as a 128-bit comparison

Ideally DAGCombiner's narrowExtractedVectorBinOp would handle this but its tricky to confirm when a target opcode can be safely extracted and performed as a different vector type

Partially improves an outstanding regression in #82290