[ARM] MVE VPNOT

This adds the patterns required to transform xor P0, -1 to a VPNOT. The
instruction operands have to change a little for this, adding an in and an out
VCCR reg and using a custom DecodeMVEVPNOT for the decode.

Differential Revision: https://reviews.llvm.org/D65133

llvm-svn: 367192