[X86][NFC] Minor improvement in X86InstrInfo::optimizeCompareInstr

Before this patch, the code enumerated `getCondFromBranch`, `getCondFromSETCC` and `getCondFromFromCMov` to get the condition code of a `MachineInstr`, and assigned the result to variable `OldCC` when `MI || IsSwapped || ImmDelta != 0` was satisfiled.

After this patch, the `if-else` structure is eliminated by using `getCondFromMI`. Since `OldCC` is only used when  `MI || IsSwapped || ImmDelta != 0`  is true, it is initialized with `getCondFromMI` directly outside the scope of `if` now.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D138349