compute: fix search pattern modification for case insensitivity

Created by: rvantonder

Fixes https://github.com/sourcegraph/sourcegraph/issues/30140.

Since Sourcegraph search queries implicitly make the search pattern case-insensitive, this needs to be mirrored in the compute world with (?i<pattern>). Previously the compute world ensured the pattern was case insensitive with (?i<pattern>) and then searched this pattern directly. This isn't valid--Sourcegraph search will do its own implicit tack-on of (?i<pattern).

This PR fixes it so that the compute world mirrors the behavior implicitly as well, at the point when it's ready to process results.