[ARM] Rewrite how VCMP are lowered, using a single node

This removes the VCEQ/VCNE/VCGE/VCEQZ/etc nodes, just using two called VCMP and
VCMPZ with an extra operand as the condition code. I believe this will make
some combines simpler, allowing us to just look at these codes and not the
operands. It also helps fill in a missing VCGTUZ MVE selection without adding
extra nodes for it.

Differential Revision: https://reviews.llvm.org/D65072

llvm-svn: 366934