[SystemZ] Add NoMerge MIFlag

Summary:
This fixes ASan and MSan tests on SystemZ after
commit 6a822e20 ("[ASan][MSan] Remove EmptyAsm and set the CallInst
to nomerge to avoid from merging.").

Based on commit 80e107cc ("Add NoMerge MIFlag to avoid MIR branch
folding").

Reviewers: uweigand, jonpa

Reviewed By: uweigand

Subscribers: hiraditya, llvm-commits, Andreas-Krebbel

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82794