[X86][AVX] Ensure zero-extension of _mm256_extract_epi8 and _mm256_extract_epi16

Ensure _mm256_extract_epi8 and _mm256_extract_epi16 zero extend their i8/i16 result to i32. This matches _mm_extract_epi8 and _mm_extract_epi16.

Fix for PR27594

Differential Revision: http://reviews.llvm.org/D20468

llvm-svn: 270330