[SystemZ::TTI] Improved cost values for comparison against memory.

Single instructions exist for i8 and i16 comparisons of memory against a
small immediate.

This patch makes sure that if the load in these cases has a single user (the
ICmp), it gets a 0 cost (folded), and also that the ICmp gets a cost of 1.

Review: Ulrich Weigand
https://reviews.llvm.org/D54897

llvm-svn: 347733