[Bug tree-optimization/66851] support double reduction in parloops

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66851

--- Comment #1 from vries at gcc dot gnu.org ---
Created attachment 35963
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35963&action=edit
Tentative patch