[mlir][Affine][NFC] Return BlockArgument in AffineForOp::getInductionVar

[dcaballe]

Summary

This will avoid unnecessary casts when a BlockArgument is required.

[bondhugula]

Good catch!

mlir/include/mlir/Dialect/Affine/IR/AffineOps.td

let extraClassDeclaration = [{
     /// Defining the function type we use for building the body of affine.for.
     using BodyBuilderFn =
         function_ref<void(OpBuilder &, Location, Value, ValueRange)>;
 
     static StringRef getStepAttrName() { return "step"; }
     static StringRef getLowerBoundAttrName() { return "lower_bound"; }
     static StringRef getUpperBoundAttrName() { return "upper_bound"; }
 
-    Value getInductionVar() { return getBody()->getArgument(0); }
+    BlockArgument getInductionVar() { return getBody()->getArgument(0); }
     Block::BlockArgListType getRegionIterArgs() {
       return getBody()->getArguments().drop_front();
     }
     Operation::operand_range getIterOperands() {
       return getOperands().drop_front(getNumControlOperands());
     }
 
     // TODO: provide iterators for the lower and upper bound operands