[X86] Rewrite getOperandBias in X86BaseInfo.h to be a little more structured and update comments to be more clear about what it does. NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328136 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed