AVX-512: Optimized INSERT_SUBVECTOR for i1 vector types

ISERT_SUBVECTOR for i1 vectors may be done with shifts, when we insert into the lower part, or into the upper part, on into all-zero vector.
CONCAT_VECTORS uses ISERT_SUBVECTOR.

Differential Revision: http://reviews.llvm.org/D14815

llvm-svn: 253819