Prefer even-odd D-register pairs.

Prefer even-odd D-register pairs.

We are sometimes allocatinog from the DPair register class which
contains odd-even pairs in addition to the Q registers.

Place the Q registers first in the DPair allocation order as they can be
copied with a single instruction. The odd-even pairs should only be
allocated as a last resort.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153699 91177308-0d34-0410-b5e6-96231b3b80d8