There was a weird check in AllocateNonParameterLocal which looked
like ".result" was treated differently from other
temporaries. This couldn't be generalized to other temporaries,
since some temporaries were both in temps_ and params_ (and some,
like ".result" would be only in params_).

Side product: Don't use AstValueFactory in scope analysis. It's
unnecessary (was only used for some DCHECKs which can be replaced
with more general checks). This change also ensures we don't
create new values during scope analysis.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2242783002
Cr-Commit-Position: refs/heads/master@{#38655}