AVX-512: dec/inc instructions are slow on KNL
After Alexey Volkov, I'm adding the same property for KNL, that prefers ADD/SUB instead of INC/DEC.
Added a test.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212178 91177308-0d34-0410-b5e6-96231b3b80d8
Undefine R2, R4, R6 after use.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212179 91177308-0d34-0410-b5e6-96231b3b80d8