[X86] Don't extend v16i8 non-uniform shifts to v16i32 if we have BWI. Use v16i16...

[X86] Don't extend v16i8 non-uniform shifts to v16i32 if we have BWI. Use v16i16 instead.

BWI supports shifting by word amounts. Even if VLX isn't support we can still widen to v32i16 and extract the lower half. For SKX its preferrable to not use 512-bit vector if we can.

llvm-svn: 321059