[NFC] Extract LastSplitPoint computation from SplitAnalysis to a new class

InsertPointAnalysis.

Because both split and spill hoisting want to use LastSplitPoint computation
result, extract the LastSplitPoint computation from SplitAnalysis class which
also contains a bunch of other analysises only related to split.

Differential Revision: http://reviews.llvm.org/D20027.

llvm-svn: 269248