MC CFG: When disassembly is impossible, fallback to data bytes.

MC CFG: When disassembly is impossible, fallback to data bytes.

This is the behavior of sequential disassemblers (llvm-objdump, ...),
when there is no instruction size hint (fixed-length, ...)

While there, also do some minor cleanup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188883 91177308-0d34-0410-b5e6-96231b3b80d8