[GISel] Don't fold convergent instruction across CFG

Before merging two instructions together, GISel does some sanity checks
that the folding is legal. However that check was missing that the
source of the pattern may be convergent. When the destination location
is in a different basic block, the folding is invalid.

Differential Revision: https://reviews.llvm.org/D128539

llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp

@@ -59,6 +59,10 @@ bool InstructionSelector::isObviouslySafeToFold(MachineInstr &MI,
       std::next(MI.getIterator()) == IntoMI.getIterator())
     return true;
 
+  // Convergent instructions cannot be moved in the CFG.
+  if (MI.isConvergent() && MI.getParent() != IntoMI.getParent())
+    return false;
+
   return !MI.mayLoadOrStore() && !MI.mayRaiseFPException() &&
          !MI.hasUnmodeledSideEffects() && MI.implicit_operands().empty();
 }