[X86] Use MachineInstr::findRegisterDefOperand to simplify some code in optimizeCompareInstr. NFCI
[X86] Use MachineInstr::findRegisterDefOperand to simplify some code in optimizeCompareInstr. NFCI

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@365946 91177308-0d34-0410-b5e6-96231b3b80d8

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