[mlir][affine] Fix crash in AffineApplyOp canonicalization

This test case used to crash with a failed assertion:
```
AffineExpr.cpp:659 in AffineExpr simplifyMul(AffineExpr, AffineExpr): lhs.isSymbolicOrConstant() || rhs.isSymbolicOrConstant()
```

This was caused by combining two affine maps, which created a multiplication of two non-symbols.