[x86] Lower some trunc + shuffle patterns to vpmov[q|d][b|w]

This should help in lowering the following four intrinsics:
 _mm256_cvtepi32_epi8
 _mm256_cvtepi64_epi16
 _mm256_cvtepi64_epi8
 _mm512_cvtepi64_epi8

Differential Revision: https://reviews.llvm.org/D46957

llvm-svn: 335238