Cleanup else-after-return and add an early-return to llvm-nm

The loop and error handling in checkMachOAndArchFlags didn't make sense
to me (a loop that only ever executes once? An error path that uses the
element the loop stopped at (which must always be a buffer overrun if
I'm reading that right?)... I'm confused) but I've made a guess at what
was intended.

Based on a patch by Richard Thomson to simplify boolean expressions.

llvm-svn: 233025