[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
llvm-svn: 313138