[X86] Stop LowerSIGN_EXTEND_AVX512 from creating v8i16/v16i16/v16i8 vselects...

[X86] Stop LowerSIGN_EXTEND_AVX512 from creating v8i16/v16i16/v16i8 vselects with a v8i1/v16i1 condition when BWI is not available.

Some of the tests in vector-shuffle-v1.ll would get into an infinite loop without this.

llvm-svn: 315172