[mlir] Don't assert when simplifying certain AffineExprs

Currently, `simplifyMul()` asserts that either `lhs` or `rhs` is symbolic or constant. This method is called by the overloaded `*` operator for `AffineExpr`s which leads to a crash when building a multiplication expression where neither operand is symbolic or constant.
This patch returns a `nullptr` from `simplifyMul()` to signal that the expression could not be simplified instead.

Author: ubfx

mlir/lib/IR/AffineExpr.cpp

@@ -774,7 +774,8 @@ static AffineExpr simplifyMul(AffineExpr lhs, AffineExpr rhs) {
     return getAffineConstantExpr(lhsConst.getValue() * rhsConst.getValue(),
                                  lhs.getContext());
 
-  assert(lhs.isSymbolicOrConstant() || rhs.isSymbolicOrConstant());
+  if (!lhs.isSymbolicOrConstant() && !rhs.isSymbolicOrConstant())
+    return nullptr;
 
   // Canonicalize the mul expression so that the constant/symbolic term is the
   // RHS. If both the lhs and rhs are symbolic, swap them if the lhs is a