Array.prototype.{reduce, reduceRight}: Wrong order of operations when determining initial value.

BUG=v8:3534
LOG=
R=svenpanne@chromium.org, wingo@igalia.com

Review URL: https://codereview.chromium.org/614733002

Patch from Diego Pino <dpino@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24807 ce2b1a6d-e550-0410-aec6-3dcde31c8c00