Push down filters do not push down variables referring to literals

__Steps to reproduce:__

Reproduction steps are given in this issue: influxdata/flux#1206.

Basically, assign a variable to a string literal and use that string literal in a filter that should be pushed down to storage.

__Expected behavior:__
The literal gets pushed down to storage for more efficient behavior by using the constant value in the variable.

__Actual behavior:__
The literal does not get pushed down because the planner does not recognize that it is equivalent to a string literal.

Start with the reproduction described in influxdata/flux#1206, then trace the planner's filter pushdown path into storage. Done means a filter using a variable assigned to a string literal is recognized as equivalent to the literal and pushed down for execution.

Written by the indexing model from the issue text.