Support simplifying expressions like str ~ '^foo$'

[wolffcm]

Is your feature request related to a problem or challenge?

It would be nice if predicates that look like

str ~ '^foo$'

could be rewritten as

str = 'foo'

And likewise,

str !~ '^foo$'

could be rewritten as

str != 'foo'

Describe the solution you'd like

It would be straightforward to extend the expression simplifier to do this.

Describe alternatives you've considered

This seems pretty uncontroversial so I haven't considered any alternatives.

Additional context

This is relevant to InfluxQL support in IOx: https://github.com/influxdata/influxdb_iox/issues/6902

[wolffcm]

I am currently working on this.