[SLP] Fix PR35628: Count external uses on extra reduction arguments.

Summary:
If the vectorized value is marked as extra reduction argument, its users
are not considered as external users. Patch fixes this.

Reviewers: mkuper, hfinkel, RKSimon, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41786

llvm-svn: 321993