Fix diassembler handling of rex.b when mod=00/01/10 and bbb=101. Mod=00 should ignore the base register entirely. Mod=01/10 should treat this as R13 plus displacment. Fixes PR18860.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201507 91177308-0d34-0410-b5e6-96231b3b80d8