AVX-512: Fixed a crash during legalization of <3 x i8> type

The compiler fails with assertion during legalization of SETCC for <3 x i8> operands.
The result is extended to <4 x i8> and then truncated <4 x i1>. It does not happen on AVX2, because the final result of SETCC is <4 x i32>.

Differential Revision: https://reviews.llvm.org/D34503

llvm-svn: 306242