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

llvm-svn: 355948