[X86] X86InstrInfo::optimizeCompareInstr - fix potential null dereference. 
[X86] X86InstrInfo::optimizeCompareInstr - fix potential null dereference. 

Fixes clang static-analyzer warning.

Technically the MachineInstr *Sub might still be null if we're comparing zero (IsCmpZero == true), although this probably won't happen as SrcReg2 is probably == 0.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371047 91177308-0d34-0410-b5e6-96231b3b80d8

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