Fix comparator used for looking up previously instantiated EDDisassemblers.

Now, Syntax is only used as a tie-breaker if the Arch
matches. Previously, a request for x86_64 disassembler followed by the
i386 disassembler in a single process would return the cached x86_64
disassembler. Fixes <rdar://problem/8958982>

llvm-svn: 125215