Tweak cross-class coalescing to be more aggressive when the target class is small.

The greedy register allocator has live range splitting and register class
inflation, so it can actually fully undo this join, including restoring the
original register classes.

We still don't want to do this for long live ranges, mostly because of the high
register pressure of there are many constrained live ranges overlapping.

llvm-svn: 131466