[X86][SKX] Added codegen regression test for avx512 instructions scheduling.NFC.

NFC.
 Added code gen regression tests for avx512 instructions scheduling called avx512-schedule.ll and
 avx512-shuffle-schedule.ll.
 This patch is in preparation of a larger patch of adding all SKX instruction scheduling and therefore
 the scheduling for the avx512 instructions are still missing.

Reviewers: zvi, delena, RKSimon, igorb
Differential Revision: https://reviews.llvm.org/D38035

Change-Id: I792762763127a921b9e13684b58af03646536533
llvm-svn: 314594