[AArch64] Improve cost of vector division by constant

With custom lowering for vector MULLH{S,U}, it is now profitable to
vectorize a divide by constant loop for the custom types (v16i8, v8i16,
and v4i32).  The cost if based on TargetLowering::Build{S,U}DIV which
uses a multiply by constant plus adjustment to express a divide by
constant.

Both {u,s}mull{2} are expressed as Instruction::Mul and shifts by
Instruction::AShr.

llvm-svn: 331873