[ARM] GlobalISel: Make struct arguments fail elegantly

The condition in isSupportedType didn't handle struct/array arguments
properly. Fix the check and add a test to make sure we use the fallback
path in this kind of situation. The test deals with some common cases
where the call lowering should error out. There are still some issues
here that need to be addressed (tail calls come to mind), but they can
be addressed in other patches.

llvm-svn: 300967