[ASan][MSan] Remove EmptyAsm and set the CallInst to nomerge to avoid from merging.

[ASan][MSan] Remove EmptyAsm and set the CallInst to nomerge to avoid from merging.

Summary: `nomerge` attribute was added at D78659. So, we can remove the EmptyAsm workaround in ASan the MSan and use this attribute.

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82322