Fix inverted condition in NativeRewriter

In PR #210, this condition was accidentally inverted. The only change
here is to uninvert it again.

I assume this wasn't causing any issues because most functions only have
one BasicBlock. I'm not sure the if statement makes any sense to begin
with, but it's hard to tell without an example of a function that has
multiple BasicBlocks. The surrounding loop does more than just look for
a lower constructor, it also calls `PromoteMemToReg` on some alloca
instructions, so it seems weird to exit early when the lower constructor
was found.

I'm ignoring all of these concerns for now, and only revert condition
back to what it was before PR #210, regardless of whether this behaviour
was correct or not.

llvm/lib/CheerpUtils/NativeRewriter.cpp

@@ -328,7 +328,7 @@ void CheerpNativeRewriterPass::rewriteConstructorImplementation(Module& M, Funct
 			lowerConstructor = CallInst::Create(newFunc, newArgs);
 			break;
 		}
-		if(!oldLowerConstructor)
+		if(oldLowerConstructor)
 			break;
 	}