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>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157354 91177308-0d34-0410-b5e6-96231b3b80d8