[SystemZ::TTI] i8/i16 operands extension costs revisited

Three minor changes to these extra costs:

* For ICmp instructions, instead of adding 2 all the time for extending each
  operand, this is only done if that operand is neither a load or an
  immediate.

* The operands extension costs for divides removed, because we now use a high
  cost already for the divide (20).

* The costs for lhsr/ashr extra costs removed as this did not seem useful.

Review: Ulrich Weigand
https://reviews.llvm.org/D55053

llvm-svn: 347961