Fix two PreGetPosts false positives in if/else branches

[GaryJones]

This follows on from #906 and fixes two false positives in PreGetPosts, one of which #906 made worse.

Outside a conditional, the sniff only flags set() calls. Inside an if without a main query check, though, it flagged any use of the query, so reading a value was reported as modifying the main query:

add_action( 'pre_get_posts', function( $query ) {
	if ( is_admin() ) {
		$post_type = $query->get( 'post_type' ); // Was flagged.
	}
} );

#906 extended the conditional handling to elseif, so the same started happening there too. The first commit makes the sniff flag only set() calls inside conditionals, as it already did outside them. Every warning the existing tests expect is on a set() call.

The second commit deals with an else after a negated check. Only the main query reaches this else, but its set() was flagged:

add_action( 'pre_get_posts', function( $query ) {
	if ( ! $query->is_main_query() ) {
		do_log( 'x' );
	} else {
		$query->set( 'cat', '-5' ); // Was flagged.
	}
} );

A later branch of an if chain, whether else, elseif or else if, with braces or the alternative syntax, now counts as checked when an earlier condition in the chain catches every query other than the main one. That means a negated is_main_query() check on its own or combined with ||. Two cases deliberately still warn. A positive check such as if ( $query->is_main_query() ) { … } else { $query->set(); } does, because only secondary queries reach that else, and catching changes to secondary queries is what the sniff is for; the existing tests already expect that warning. A negated check combined with && does as well, because other queries can still reach the later branches.

I ran the old and new sniff over the 21 Automattic plugins I have locally and the results did not change, though this sniff only reported four issues across them.