[X86][Skylake][KNL] Updating code gen regression test to use the KNL and SKYLAKE...

[X86][Skylake][KNL] Updating code gen regression test to use the KNL and SKYLAKE prefixes. NFC.

NFC.
Updating the code gen regression test bmi2-schedule.ll to use the KNL and SKYLAKE prefixes for the run commands that use the knl and Skylake mcpu options.
The fix is in preparation for a large patch of adding all SKL scheduling information.

Reviewers: delena, zvi, RKSimon
Revision: https://reviews.llvm.org/D37796

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313138 91177308-0d34-0410-b5e6-96231b3b80d8