Fix diassembler handling of rex.b when mod=00/01/10 and bbb=101. Mod=00 should...

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.

llvm-svn: 201507