Pruning on `!=` predicate results in incorrect results

[alamb]

Describe the bug
The logic for pruning on != predicates introduced in #544 is incorrect

To Reproduce
Use the pruning logic with a != predicate where the range covers the expression

So like s != M where the min/max is A -> Z.

Expected behavior
Must check all values in block (as s!=M might be true for every one)

Actual behavior: Pruning logic says we must look at it.

Additional context
I found this issue when integrating / testing DataFusion into IOx here: https://github.com/influxdata/influxdb_iox/pull/1721#issuecomment-860858033