[ARM] Fix AAPCS regression caused by r211898

r211898 introduced a regression where a large struct, which would
normally be passed ByVal, was causing padding to be inserted to
prevent the backend from using some GPRs, in order to follow the
AAPCS. However, the type of the argument was not being set correctly,
so the backend cannot align 8-byte aligned struct types on the stack.

The fix is to not insert the padding arguments when the argument is
being passed ByVal.