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.
Always let value types influence register classes.

When creating a virtual register for a def, the value type should be
used to pick the register class. If we only use the register class
constraint on the instruction, we might pick a too large register class.

Some registers can store values of different sizes. For example, the x86
xmm registers can hold f32, f64, and 128-bit vectors. The three
different value sizes are represented by register classes with identical
register sets: FR32, FR64, and VR128. These register classes have
different spill slot sizes, so it is important to use the right one.

The register class constraint on an instruction doesn't necessarily care
about the size of the value its defining. The value type determines
that.

This fixes a problem where InstrEmitter was picking 32-bit register
classes for 64-bit values on SPARC.