Handle subquery merging with references correctly

[systay]

Description

Make sure to merge subqueries where either the inner or the outer are reference tables.

Related Issue(s)

Fixes #8638

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[frouioui]

in TestSubqueryCanMerge, the testcases matrix should be:

	testcases := [engine.NumRouteOpcodes][engine.NumRouteOpcodes]bool{
		{true, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, false, true, false},
		{false, false, false, false, false, false, true, true, false},
		{true, true, true, true, true, true, true, true, true},
		{false, false, false, false, false, false, false, true, false},
	}

[harshit-gangal]

this needs to be fixed in all releases and main.