[X86] getFauxShuffleMask - recognise CONCAT(SUB0, SUB1) style patterns

Handles the INSERT_SUBVECTOR(INSERT_SUBVECTOR(UNDEF,SUB0,0),SUB1,N) pattern

Currently limited to v8i64/v8f64 cases as only AVX512 has decent cross lane 2-input shuffles, the plan is to relax this as I deal with some regressions