[mlir] Toy tutorial: insert terminators at the end of the loop during rewrite

[ftynse]

Summary

When creating temporary scf.for loops in toy.print lowering, the block
insertion point was erronously set up to the beginning of the block rather than
to its end, contradicting the comment just above the insertion point change.
The code was nevertheless operational because scf.for was setting up its
scf.yield terminator in an opaque to the pattern rewriting infrastructure
way. Now that it is about to change, the problem would have been exposed and
lead to conversion failures.

mlir/examples/toy/Ch6/mlir/LowerToLLVM.cpp

for (unsigned i = 0, e = memRefShape.size(); i != e; ++i) {
       auto upperBound = rewriter.create<ConstantIndexOp>(loc, memRefShape[i]);
       auto step = rewriter.create<ConstantIndexOp>(loc, 1);
       auto loop =
           rewriter.create<scf::ForOp>(loc, lowerBound, upperBound, step);
       loop.getBody()->clear();
       loopIvs.push_back(loop.getInductionVar());
 
       // Terminate the loop body.
-      rewriter.setInsertionPointToStart(loop.getBody());
+      rewriter.setInsertionPointToEnd(loop.getBody());
 
       // Insert a newline after each of the inner dimensions of the shape.
       if (i != e - 1)
         rewriter.create<CallOp>(loc, printfRef, rewriter.getIntegerType(32),
                                 newLineCst);
       rewriter.create<scf::YieldOp>(loc);
       rewriter.setInsertionPointToStart(loop.getBody());
     }

mlir/examples/toy/Ch7/mlir/LowerToLLVM.cpp

for (unsigned i = 0, e = memRefShape.size(); i != e; ++i) {
       auto upperBound = rewriter.create<ConstantIndexOp>(loc, memRefShape[i]);
       auto step = rewriter.create<ConstantIndexOp>(loc, 1);
       auto loop =
           rewriter.create<scf::ForOp>(loc, lowerBound, upperBound, step);
       loop.getBody()->clear();
       loopIvs.push_back(loop.getInductionVar());
 
       // Terminate the loop body.
-      rewriter.setInsertionPointToStart(loop.getBody());
+      rewriter.setInsertionPointToEnd(loop.getBody());
 
       // Insert a newline after each of the inner dimensions of the shape.
       if (i != e - 1)
         rewriter.create<CallOp>(loc, printfRef, rewriter.getIntegerType(32),
                                 newLineCst);
       rewriter.create<scf::YieldOp>(loc);
       rewriter.setInsertionPointToStart(loop.getBody());
     }