[X86] Only pass v64i8/v32i16 as v16i32 on non-avx512bw targets if the v16i32 type...

[X86] Only pass v64i8/v32i16 as v16i32 on non-avx512bw targets if the v16i32 type won't be split by prefer-vector-width=256

Otherwise just let the v64i8/v32i16 types be split to v32i8/v16i16.

In reality this shouldn't happen because it means we have a 512-bit
vector argument, but min-legal-vector-width says a value less than
512. But a 512-bit argument should have been factored into the
preferred vector width.