[test] Fix use of 'sort -b' in SimpleLoopUnswitch on NetBSD

Add '-k 1' to 'sort -b' calls in SimpleLoopUnswitch tests, as required
for sort implementation on NetBSD.  The '-b' modifier is ineffective
if specified without any key.  Per the manpage:

  Note that the -b option has no effect unless key fields are specified.

Differential Revision: https://reviews.llvm.org/D55168

llvm-svn: 348097