Fix assertion in jump threading (PR13405).

GetBestDestForJumpOnUndef() assumes there is at least 1 successor, which isn't
true if the block ends in an indirect branch with no successors. Fix this by
bailing out earlier in this case.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160546 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/JumpThreading/2012-07-19-NoSuccessorIndirectBr.ll

@@ -0,0 +1,8 @@
+; RUN: opt < %s -jump-threading
+; PR 13405
+; Just check that it doesn't crash / assert
+
+define i32 @f() nounwind {
+entry:
+  indirectbr i8* undef, []
+}