ARM: remove unused v(add|sub)hn and vqdml[as]l intrinsics.

Clang is now generating cleaner IR, so this removes the old variants which
should be completely unused.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189481 91177308-0d34-0410-b5e6-96231b3b80d8
[ARMv8] Add a missing IsThumb to t2LDAEXD.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189482 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: Use "dmb sy" for barriers on M-class CPUs

The usual default of "dmb ish" (inner-shareable) isn't even a valid instruction
on v6M or v7M (well, it does the same thing but software is strongly
discouraged from using it) so we should emit a full-system barrier there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189483 91177308-0d34-0410-b5e6-96231b3b80d8