Add a last resort tryInstructionSplit() to RAGreedy.

Add a last resort tryInstructionSplit() to RAGreedy.

Live ranges with a constrained register class may benefit from splitting
around individual uses. It allows the remaining live range to use a
larger register class where it may allocate. This is like spilling to a
different register class.

This is only attempted on constrained register classes.

<rdar://problem/11438902>

llvm-svn: 157354