[SystemZ]  NFC: use clearRegisterDeads() in SystemZElimCompare.cpp
[SystemZ]  NFC: use clearRegisterDeads() in SystemZElimCompare.cpp

This is simpler than using findRegisterDefOperandIdx() + setIsDead().

Review: Ulrich Weigand.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371369 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm