[llvm] [SCEV] Fix a hang introduced by collectForPHI (PR #158153)

[Philip Reames via llvm-commits]

================
@@ -364,3 +364,34 @@ body:
 exit:
   ret void
 }
+
+define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
----------------
preames wrote:

Oh, I see what happened.  The original test definite hungs, but when I reduced this via llvm-reduce, I used a proxy assert (the one in collectForBlock).  I didn't manually add back the self-loop block on the unreachable path.  Let me try to do that to confirm this actually does hang as opposed to simply break the suggested invariant.  

https://github.com/llvm/llvm-project/pull/158153