Fix a bug found by inspection by PVS-Studio.

This condition is trivially always true prior to the change. The comment
at the call site makes it clear that we expect *all* of these to be '=',
'S', or 'I' so fix the code.

We have a bug I will update to track the fact that Clang doesn't warn on
this: http://llvm.org/PR13101

llvm-svn: 285930