Switch the NEON register class from QPR to DPair.

Switch the NEON register class from QPR to DPair.

The already allocatable DPair superclass contains odd-even D register
pair in addition to the even-odd pairs in the QPR register class. There
is no reason to constrain the set of D register pairs that can be used
for NEON values. Any NEON instructions that require a Q register will
automatically constrain the register class to QPR.

The allocation order for DPair begins with the QPR registers, so
register allocation is unlikely to change much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199186 91177308-0d34-0410-b5e6-96231b3b80d8