AArch64: mark small types (i1, i8, i16) as promoted

This means the output of LowerFormalArguments returns a lowered
SDValue with the correct type (expected in SelectionDAGBuilder).
Without this, an assertion under a DEBUG macro triggers when those
types are passed on the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210102 91177308-0d34-0410-b5e6-96231b3b80d8