fix: while validating logql expression, detect and validate expression in label_replace

[sandeepsukhani]

What this PR does / why we need it:
There was a panic due to an invalid query, which looked something like this:
sum(label_replace(count_over_time({container="query-frontend"} |= "metrics.go" | logfmt[1m]) or 0, "tenant", "$1", "org_id", "(.*)")) by (tenant)

Notice the or 0 in the binary operation, which is invalid and results in setting an error in the initialised BinOpExpr.

Now, we validate the parsed logQL query to detect any such errors and return them to the user.
Some expressions have specific validation logic, while for the remaining, we default to calling the Selector method on that expression to see if there is an issue.
The thing to note is that we do not have specific validation logic for label_replace. Which means, when the above expression is being validated, we not only miss running the validation logic for binary operation, but we also end up returning a panic here since BinOpExpr does not implement Selector method and relies on embedded SampleExpr, which would be nil when there is invalid binary expression like above.