AArch64: force i1 to be zero-extended at an ABI boundary.

This commit is debatable. There are two possible approaches, neither
of which is really satisfactory:

1. Use "@foo(i1 zeroext)" to mean an extension to 32-bits on Darwin,
   and 8 bits otherwise.
2. Redefine "@foo(i1)" to mean that the i1 is extended by the caller
   to 8 bits. This goes against the spirit of "zeroext" I think, but
   it's a bit of a vague construct anyway (by definition you're going
   to extend to the amount required by the ABI, that's why it's the
   ABI!).

This implements option 2. The DAG machinery really isn't setup for the
first (there's a fairly strong assumption that "zeroext" goes to at
least the smallest register size), and even if it was the resulting
DAG looks like it would be inferior in many cases.

Theoretically we could add AssertZext nodes in the consumers of
ABI-passed values too now, but this actually seems to make the code
worse in practice by making truncation proceed in two steps. The code
produced is equally valid if we continue to assume only the low bit is
defined.

Should fix PR19850

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209637 91177308-0d34-0410-b5e6-96231b3b80d8

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