InstCombine: fix sinking of convergent calls

[majnemer]

Might be nice to share the logic with Sink.cpp, I guess it would hinge on
an appropriate name for a predicate.

[escha]

Ping?

I think if we're going to merge the logic here we should probably do the refactoring separately from any correctness changes.

[majnemer]

LGTM.

lib/Transforms/InstCombine/InstructionCombining.cpp

         &DestBlock->getParent()->getEntryBlock())
     return false;
 
+  // Do not sink convergent call instructions.
+  if (auto *CI = dyn_cast<CallInst>(I)) {
+    if (CI->isConvergent())
+      return false;
+  }
+
   // We can only sink load instructions if there is nothing between the load and
   // the end of block that could change the value.
   if (I->mayReadFromMemory()) {