Lazily defer duplicating the live interval we are splitting until we know it is

Lazily defer duplicating the live interval we are splitting until we know it is
necessary.

Sometimes, live range splitting doesn't shrink the current interval, but simply
changes some instructions to use a new interval. That makes the original more
suitable for spilling. In this case, we don't need to duplicate the original.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110481 91177308-0d34-0410-b5e6-96231b3b80d8