[X86] Add getAVX512Node helper. NFC.

For AVX512 targets without VLX, we have to widen 128/256-bit vectors to 512-bits to use some specific AVX512 instructions (or some other instructions with predicates etc.).

I've pulled out the widening code from LowerFunnelShift into the helper function, so we can convert some other widening patterns in the future.