Reassociate mixture of add and sub to wide trees

[KGrewal1]

As in #7466 , converts deep nesting of addition and subtraction into a wide tree eg:

$((a-b)-c)-d)=(a-b)-(c+d)$

again using subsume to prevent blow up due to reassociating large chains

[github-actions]

Subscribe to Label Action

cc @cfallin, @fitzgen

This issue or pull request has been labeled: "cranelift", "isle"

Thus the following users have been cc'd because of the following labels:

• cfallin: isle
• fitzgen: isle

To subscribe or unsubscribe from this label, edit the .github/subscribe-to-label.json configuration file.

Learn more.

[abrown]

Looks fine to me but does anyone with ISLE conventions paged in want to comment: @cfallin, @elliottt, @jameysharp?

[fitzgen]

LGTM! Thanks!

[fitzgen]

(Some network issue when installing open-vino in CI; retrying)