Partial fix for #15048: shiftNegative for conditional value

[chrchr-github]

No description provided.

[danmar]

This is an AI review. Take it with a grain of salt and feel free to reject it by resolving the comment.

getErrorMessages() calls this with v == nullptr, so --errorlist now reports shiftNegative as severity="warning" instead of severity="error". I checked this with a build of the PR. Tools that use the error list (GUI, docs, severity mappings) would see the downgrade even though a known negative shift is still an error. Maybe treat a missing value as the non-conditional case:

Suggested change

	const Severity severity = isLHS ? Severity::portability : (v && v->errorSeverity() && !v->conditional ? Severity::error : Severity::warning);
	const Severity severity = isLHS ? Severity::portability : (!v || (v->errorSeverity() && !v->conditional) ? Severity::error : Severity::warning);

Apart from that, the new severity matches how zerodiv/zerodivcond handle the same patterns. I compared with int d = 0; if (b) return i; return i / d;.