[SystemZ::TTI] Improve cost for compare of i64 with extended i32 load

CGF/CLGF compares an i64 register with a sign/zero extended loaded i32 value
in memory.

This patch makes such a load considered foldable and so gets a 0 cost.

Review: Ulrich Weigand
https://reviews.llvm.org/D54944

llvm-svn: 347735