[X86][AVX512] Detect repeated constant patterns in BUILD_VECTOR suitable for broadcasting.

Check if a build_vector node includes a repeated constant pattern and replace it with a broadcast of that pattern.
For example:
"build_vector <0, 1, 2, 3, 0, 1, 2, 3>" would be replaced by "broadcast <0, 1, 2, 3>"

Differential Revision: https://reviews.llvm.org/D26802

llvm-svn: 288804