[SystemZ::TTI] Return zero cost for ICmp that becomes Load And Test.

A loaded value with multiple users compared with 0 will become a load and
test single instruction. The load is not folded in this case (multiple
users), but the compare instruction is eliminated.

This patch returns 0 cost for the icmp in these cases.

Review: Ulrich Weigand
https://reviews.llvm.org/D55111

llvm-svn: 348141