[Fix #438] Fix an incorrect autocorrect for Performance/ConstantRegex…

[pcbeingused333]

…p in pattern matching

Fixes #438.

Performance/ConstantRegexp autocorrects an interpolated-constant regexp by appending the /o option. When the regexp is used as a pattern in pattern matching (case/in, one-line in), /o is invalid Ruby and the autocorrection produces broken code (NODE_ONCE: unknown node).

This skips registering an offense when the regexp is in a pattern-matching pattern position, mirroring the existing within_allowed_assignment? guard. Added specs for the case/in and one-line in forms.

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