ARM: use a more precise check for MachO

Rather than relying on support for a specific directive to determine if we are
targeting MachO, explicitly check the output format.

As an additional bonus, cleanup the caret diagnostic for the non-MachO case and
avoid the spurious error caused by not discarding the statement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218012 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Initial step of teaching the new vector shuffle lowering about
PALIGNR. This just adds it to the v8i16 and v16i8 lowering steps where
it is completely unmatched. It also introduces the logic for detecting
rotation shuffle masks even in the presence of single input or blend
masks and arbitrarily undef lanes.

I've added fairly comprehensive tests for the matching logic in v8i16
because the tests at that size are much easier to write and manage.

I've not checked the SSE2 code generated for these tests because the
code is *horrible*. It is absolute madness. Testing it will just make
the test brittle without giving any interesting improvements in the
correctness confidence.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218013 91177308-0d34-0410-b5e6-96231b3b80d8