Performance: use IsSingleShard() check in pushDerived instead of just engine.EqualUnique opcode

[ChaitanyaD48]

Description

This PR enhances the pushDerived function in route_planning.go to consider all single-shard routing scenarios, not just those using the engine.EqualUnique opcode.

IsSingleShard() method checks if a route is a single shard route by looking at its opcode. The method returns true for the following opcodes: Unsharded, DBA, Next, EqualUnique, and Reference.

Reference - vitess/go/vt/vtgate/engine/routing.go

func (code Opcode) IsSingleShard() bool {
	switch code {
	case Unsharded, DBA, Next, EqualUnique, Reference:
		return true
	}
	return false
}

Related Issue(s)

Fixes #18952

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[arthurschreiber]

Can you also add test cases that show the impact of this change?