Drop AffineExpr::Null

Similarly to other value-type wrappers, the default constructor of AffineExpr
    constructs a null object and removes the need for an explicit ::Null
    constructor.  Drop it and remove the only user which can trivially rely on the
    default constructor.

--

PiperOrigin-RevId: 249207502

mlir/include/mlir/IR/AffineExpr.h

@@ -71,7 +71,7 @@ enum class AffineExprKind {
 /// Base type for affine expression.
 /// AffineExpr's are immutable value types with intuitive operators to
 /// operate on chainable, lightweight compositions.
-/// An AffineExpr is a POD interface to the underlying storage type pointer.
+/// An AffineExpr is an interface to the underlying storage type pointer.
 class AffineExpr {
 public:
   using ImplType = detail::AffineExprStorage;
@@ -86,8 +86,6 @@ public:
     return *this;
   }
 
-  static AffineExpr Null() { return AffineExpr(nullptr); }
-
   bool operator==(AffineExpr other) const { return expr == other.expr; }
   bool operator!=(AffineExpr other) const { return !(*this == other); }
   explicit operator bool() const { return expr; }

mlir/lib/Analysis/AffineStructures.cpp

@@ -1478,7 +1478,7 @@ void FlatAffineConstraints::getSliceBounds(unsigned num, MLIRContext *context,
   LLVM_DEBUG(dump());
 
   // Record computed/detected identifiers.
-  SmallVector<AffineExpr, 8> memo(getNumIds(), AffineExpr::Null());
+  SmallVector<AffineExpr, 8> memo(getNumIds());
   // Initialize dimensional and symbolic identifiers.
   for (unsigned i = num, e = getNumDimIds(); i < e; i++)
     memo[i] = getAffineDimExpr(i - num, context);
@@ -1565,7 +1565,7 @@ void FlatAffineConstraints::getSliceBounds(unsigned num, MLIRContext *context,
 
   // Set the lower and upper bound maps for all the identifiers that were
   // computed as affine expressions of the rest as the "detected expr" and
-  // "detected expr + 1" respectively; set the undetected ones to Null().
+  // "detected expr + 1" respectively; set the undetected ones to null.
   Optional<FlatAffineConstraints> tmpClone;
   for (unsigned pos = 0; pos < num; pos++) {
     unsigned numMapDims = getNumDimIds() - num;