Order register classes by spill size first, members last.

This is still a topological ordering such that every register class gets
a smaller enum value than its sub-classes.

Placing the smaller spill sizes first makes a difference for the
super-register class bit masks. When looking for a super-register class,
we usually want the smallest possible kind of super-register. That is
now available as the first bit set in the bit mask.

llvm-svn: 156222