[Headers] Add tests for _mm256_insert_epi64 and fix its definition

Summary:
The definition for _mm256_insert_epi64 was taking an int, which would get
truncated before being inserted in the vector.

Original patch by Joshua Magee!

Reviewers: bruno, craig.topper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7179

llvm-svn: 229811