Extend loop unrolling to unroll by a given factor; add builder for affine
apply op.

- add builder for AffineApplyOp (first one for an operation that has
  non-zero operands)
- add support for loop unrolling by a given factor; uses the affine apply op
  builder.

While on this, change 'step' of ForStmt to be 'unsigned' instead of
AffineConstantExpr *. Add setters for ForStmt lb, ub, step.

Sample Input:

// CHECK-LABEL: mlfunc @loop_nest_unroll_cleanup() {
mlfunc @loop_nest_unroll_cleanup() {
  for %i = 1 to 100 {
    for %j = 0 to 17 {
      %x = "addi32"(%j, %j) : (affineint, affineint) -> i32
      %y = "addi32"(%x, %x) : (i32, i32) -> i32
    }
  }
  return
}

Output:

$ mlir-opt -loop-unroll -unroll-factor=4 /tmp/single2.mlir
#map0 = (d0) -> (d0 + 1)
#map1 = (d0) -> (d0 + 2)
#map2 = (d0) -> (d0 + 3)
mlfunc @loop_nest_unroll_cleanup() {
  for %i0 = 1 to 100 {
    for %i1 = 0 to 17 step 4 {
      %0 = "addi32"(%i1, %i1) : (affineint, affineint) -> i32
      %1 = "addi32"(%0, %0) : (i32, i32) -> i32
      %2 = affine_apply #map0(%i1)
      %3 = "addi32"(%2, %2) : (affineint, affineint) -> i32
      %4 = affine_apply #map1(%i1)
      %5 = "addi32"(%4, %4) : (affineint, affineint) -> i32
      %6 = affine_apply #map2(%i1)
      %7 = "addi32"(%6, %6) : (affineint, affineint) -> i32
    }
    for %i2 = 16 to 17 {
      %8 = "addi32"(%i2, %i2) : (affineint, affineint) -> i32
      %9 = "addi32"(%8, %8) : (i32, i32) -> i32
    }
  }
  return
}

PiperOrigin-RevId: 209676220
diff --git a/include/mlir/IR/Builders.h b/include/mlir/IR/Builders.h
index 62c0dcb..e8d3894 100644
--- a/include/mlir/IR/Builders.h
+++ b/include/mlir/IR/Builders.h
@@ -252,7 +252,8 @@
     this->insertPoint = insertPoint;
   }
 
-  /// Set the insertion point to the specified operation.
+  /// Set the insertion point to the specified operation, which will cause
+  /// subsequent insertions to go right before it.
   void setInsertionPoint(Statement *stmt) {
     setInsertionPoint(stmt->getBlock(), StmtBlock::iterator(stmt));
   }
@@ -298,8 +299,7 @@
 
   // Creates for statement. When step is not specified, it is set to 1.
   ForStmt *createFor(AffineConstantExpr *lowerBound,
-                     AffineConstantExpr *upperBound,
-                     AffineConstantExpr *step = nullptr);
+                     AffineConstantExpr *upperBound, int64_t step = 1);
 
   IfStmt *createIf(IntegerSet *condition) {
     auto *stmt = new IfStmt(condition);