[PowerPC] Handle loop predecessor invokes

[PowerPC] Handle loop predecessor invokes

If a loop predecessor has an invoke as its terminator, and the return value
from that invoke is used to determine the loop iteration space, then we can't
insert a computation based on that value in the loop predecessor prior to the
terminator (oops). If there's such an invoke, or just no predecessor for that
matter, insert a new loop preheader.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228488 91177308-0d34-0410-b5e6-96231b3b80d8