[X86][SKX][KNL] Updating several CodeGen tests to use the attr flag instead of mcpu flag

NFC.
 Updated 3 Codegen regression tests to use the -mattr flag instead of the -mcpu flags as follows:
 Instead of -mcpu=skx use -mattr=+avx512f,+avx512bw,+avx512vl,+avx512dq
 Instead of -mcpu=knl use -mattr=+avx512f

Reviewers: delena
Revision: https://reviews.llvm.org/D37674
llvm-svn: 312909