use if matches! instead of match for simplicity

[GrantBirki]

This pull request updates the check.rs + assert.rs files to use matches! instead of match for simplicity. This change makes it a bit easier to read without changing any core functionality.

[rustbot]

r? @petrochenkov

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[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to constck

cc @fee1-dead

[RalfJung]

I don't think that const eval change improves readability. If anything I find the old code more clear than the new code.

[lcnr]

I agree with @RalfJung that this worsens readability. Even if others consider this change to be neutral, it would not be worth the churn.

Thank you for your contribution and we'd be happy to review more substantial changes in the future