[SLP] Fix invalid triple in X86 tests

x86-64 is an invalid architecture in triples. Changing it to the correct
triple (x86_64) changes some tests, because SLP is not deemed profitable
any more.

Reviewers: ABataev, RKSimon, spatel

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D58931

llvm-svn: 355420