Re-land r360859: "[MergeICmps] Simplify the code."

With a fix for PR41917: The predecessor list was changing under our feet.

-  for (BasicBlock *Pred : predecessors(EntryBlock_)) {
+  while (!pred_empty(EntryBlock_)) {
+    BasicBlock* const Pred = *pred_begin(EntryBlock_);

llvm-svn: 361009