_mm_extract_epi16: use "& 7" when index is out of bound.

This is in line with implementation of _mm_extract_pi16.
rdar://15250497


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193187 91177308-0d34-0410-b5e6-96231b3b80d8