[X86] Modify the conditions for when we stop making v16i8/v32i8 rotate Custom...

[X86] Modify the conditions for when we stop making v16i8/v32i8 rotate Custom based on having avx512 features.

The comments here indicate that we prefer to promote the shifts
instead of allowing rotate to be pattern matched. But we weren't
taking into account whether 512-bit registers are enabled or
whethever we have vpsllvw/vpsrlvw instructions.

splatvar_rotate_v32i8 is a slight regrssion, but the other cases
are neutral or improved.