Fix SCEVExpander's canonical addrec expansion code to work on loops that

Fix SCEVExpander's canonical addrec expansion code to work on loops that
aren't in canonical loop-simplify form, since it doesn't itself depend
on LoopSimplify. This means handling loops without preheaders and loops
with multiple backedges.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82905 91177308-0d34-0410-b5e6-96231b3b80d8