[SystemZ] Improve foldMemoryOperandImpl()

A spilled load of an immediate can use MVHI/MVGHI instead.
A compare of a spilled register against an immediate can use CHSI/CGHSI.
A logical compare can use CLFHSI/CLGHSI.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D76055