[X86] Fix a bad cast from mask16 to mask8 in _mm256_mask_cvtepi16_epi8 introduced in r332266.
[X86] Fix a bad cast from mask16 to mask8 in _mm256_mask_cvtepi16_epi8 introduced in r332266.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@332738 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang