ARM: correct toggling behaviour

This was a thinko.  The intent was to flip the explicit bits that need toggling
rather than all bits.  This would result in incorrect behaviour (which now is
tested).

Thanks to Nico Weber for pointing this out!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215846 91177308-0d34-0410-b5e6-96231b3b80d8