[Disassembler] Simplify a few methods (2/2) (NFC)
[Disassembler] Simplify a few methods (2/2) (NFC)

Use early returns to highlight preconditions and make the code easier to
follow.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@370998 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb