Set useful flags for vector imm setting instructions

Vector imm setting instructions like XXLXORz/XXLXORspz/XXLXORdpz
Should behave like LI8.

We should set corresponding flags to allow rematerialization and other
opts in LICM, RA, Scheduling etc.

Differential Revision: https://reviews.llvm.org/D58645

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355948 91177308-0d34-0410-b5e6-96231b3b80d8