Optimize `needless_bool` lint (#15423)

Two optimizations have been done when checking for the context in which
to apply the lint:

- Checking for the mere presence of comments does not require building a
`String` with the comment to then check if it is empty and discard it.
- Checking for the presence of comment can be done after we have checked
that we do have a `if` construct that we intend to lint instead of for
every expression.

changelog: none

r? blyxyas

0 files changed, 0 insertions, 0 deletions