Enhanced heuristic to determine the *best* register to spill. Instead of...

Enhanced heuristic to determine the *best* register to spill. Instead of picking the register with the lowest spill weight. Consider (up to) 2 additional registers with spill weights that are close to the lowest spill weight. The one with fewest defs and uses that conflicts with the current interval (weighted by loop depth) is the spill candidate.

This is not always a win, but there are much more wins than loses and wins tend to be more noticeable.

llvm-svn: 52554