The cache entry referenced by the best split candidate could become
clobbered by an unsuccessful candidate.

The correct fix here is to use reference counts on the cache entries.
Coming up.

llvm-svn: 135113

llvm-svn: 135096

This is in preparation of supporting multiple global split candidates in
a single live range split operation.

llvm-svn: 135074

This doesn't require seeking in the live interval union, so it is very cheap.

llvm-svn: 129187

When the greedy register allocator is splitting multiple global live ranges, it
tends to look at the same interference data many times. The InterferenceCache
class caches queries for unaltered LiveIntervalUnions.

llvm-svn: 128764