ARM & AArch64: merge NEON absolute compare intrinsics

There was an extremely confusing proliferation of LLVM intrinsics to implement
the vacge & vacgt instructions. This combines them all into two polymorphic
intrinsics, shared across both backends.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200768 91177308-0d34-0410-b5e6-96231b3b80d8