Disallow discarded EqualExpression for a struct with `opEquals`

[ntrel]

The following errors:

	int x;
	x == 1;

structopeq.d(15): Error: the result of the equality expression x == 1 is discarded

It also errors for x++ == 1;, even though that has a side-effect.

This could be extended for EqualExpression when lowering to opEquals:

struct S
{
	bool opEquals(int);
}

void main()
{
	S s;
	s == 1; // currently allowed, should error
}

Similar to the PR to disallow discarded assignment, this should still error even if opEquals may have a side-effect.