[x86] Fix uninitialized variable warning in translate{Src,Dst}Index

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199811 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Silence unused diReg variable warning in non-asserting builds

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199812 91177308-0d34-0410-b5e6-96231b3b80d8