[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
ARM: prevent crash on ELF directives on COFF

Certain directives are unsupported on Windows (some of which could/should be
supported).  We would not diagnose the use but rather crash during the emission
as we try to access the Target Streamer.  Add an assertion to prevent creating a
NULL reference (which is not permitted under C++) as well as a test to ensure
that we can diagnose the disabled directives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218014 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Add an SSSE3 run to the v4 shuffle test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218028 91177308-0d34-0410-b5e6-96231b3b80d8