[AVX512] Fix insertelement i1 lowering.

1. Use shuffle to insert element i1 into vector. The previous implementation was incorrect ( dest_bit OR src_bit , it doesn't clear the bit if src_bit=0 )
2. Improve shuffle i1 vector, use CVT2MASK if supported instead TRUNCATE.

Differential Revision: http://reviews.llvm.org/D23347

llvm-svn: 278623