AVX-512: Changed lowering of BITCAST between i1 vectors and i8/i16/i32 integer values

Optimized lowering of BITCAST node. The BITCAST node can be replaced with COPY_TO_REG instead of KMOV.
It allows to suppress two opposite BITCAST operations and avoid redundant "movs".

Differential Revision: https://reviews.llvm.org/D23247

llvm-svn: 277958