[ARM] MVE integer compares and selects

This adds the very basics for MVE vector predication, adding integer VCMP and
VSEL instruction support. This is done through predicate registers (MVT::v16i1,
MVT::v8i1, MVT::v4i1), but otherwise using same mechanics as NEON to custom
lower setcc's through ARMISD::VCXX nodes (VCEQ, VCGT, VCEQZ, etc).

An extra VCNE was added, as this can be handled sensibly by MVE's expanded
number of VCMP condition codes. (There are also VCLE and VCLT which are added
later).

VPSEL is also added here, simply selecting on the vselect.

Original code by David Sherwood.

Differential Revision: https://reviews.llvm.org/D65051

llvm-svn: 366885