This additionally optimizes ExpressionListToExpression in the parser to
allocate Nary if possible.  This allows us to drop unnecessary intermediate
objects in the parser, and avoids all the work altogether in the preparser.

Change-Id: I4a7d0ec3a28624c94ed85959d291e54eb81ffce3
Reviewed-on: https://chromium-review.googlesource.com/c/1297952
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56943}