[AVX-512] Always use v8i64 when converting 512-bit FAND/FOR/FXOR/FANDN to...

[AVX-512] Always use v8i64 when converting 512-bit FAND/FOR/FXOR/FANDN to integer operations when DQI isn't supported. This is consistent with the recent changes to promote logical operations to i64 vectors.

llvm-svn: 279957