[X86][Skylake] Fixing duplicated prefixes in the run command of Code Gen regression tests

NFC.
Replaced duplicated HASWELL prefixes in run commands in the X86 Code Gen regression tests by the SKYLAKE prefix when the -mcpu is set to skylake.
The fix is needed in preparation of an upcoming patch containing the Skylake scheduling info.

Reviewers: zvi, RKSimon, aymanmus, igorb

Differential Revision: https://reviews.llvm.org/D37258

llvm-svn: 312103