B::Deparse: retain () around ! if omitting them would warn

[mauke]

Since 570fa43328, we emit precedence warnings if the LHS of a comparison or binding op starts with an unparenthesized logical negation (!, as in !$x == $y). Explicit parens can be used to avoid the warning ((!$x) == $y).

Teach B::Deparse to keep these parentheses even if they're not strictly required by operator precedence because we don't want the deparsed code to generate more warnings than the original code.

Fixes #22661.

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