PrologueEpilogInserter: Fix missing live-ins when savepoint equals restorepoint

The algorithm would not modify the live-in list of blocks below the save
block point which is correct unless it happens to be a restore point at
the same time.
Also fixes the benign issue of live-in registers being added twice in
some cases.

The testcase is based on a test submitted by Kit Barton.

Differential Revision: http://reviews.llvm.org/D13176

llvm-svn: 248620