[GuardWidening] Fix incorrect use of remove_if

I had used `std::remove_if` under the assumption that it moves the
predicate matching elements to the end, but actaully the elements
remaining towards the end (after the iterator returned by
`std::remove_if`) are indeterminate.  Fix the bug (and make the code
more straightforward) by using a temporary SmallVector, and add a test
case demonstrating the issue.

llvm-svn: 270306