Make the _mm256_insert_epi64 definition more consistent

Use long long for the epi64 argument, like the other intrinsics.
NFC since this is only defined in 64-bit mode, not in 32-bit.

Fix suggested by H. J. Lu!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229886 91177308-0d34-0410-b5e6-96231b3b80d8