Multiplication by multiple constant values don't merge correctly

[SyfSchydea]

Doing a multiplication such as 3 * 5 * x expands correctly, but multiplications such as x * 3 * 5 do not. Multiplication between constants should be expanded first, before multiplications between expressions and constants.

Note that removing the expr.has_side_effects() guard on the variable assignment in validate_expr_mul_const is not sufficient to solve this problem. In cases such as 17 * 3, where the multiplication expansion is not the same as the multiplication written, suboptimal code will still be produced. (In this case, x*51 expands to x * 5 * 5 * 2 + x)