[SIL] Implement a very simple isLegalToHoistInto for llvm::LoopInfo.

I'm not a SIL author, so it's likely that this could be improved,
but I think this is closest to the old LLVM behavior.

lib/SIL/SILBasicBlock.cpp

@@ -355,3 +355,7 @@ bool SILBasicBlock::isTrampoline() const {
     return false;
   return begin() == Branch->getIterator();
 }
+
+bool SILBasicBlock::isLegalToHoistInto() const {
+  return true;
+}