AVX-512: Gather and Scatter patterns

Gather and scatter instructions additionally write to one of the source operands - mask register.
In this case Gather has 2 destination values - the loaded value and the mask.
Till now we did not support code gen pattern for gather - the instruction was generated from 
intrinsic only and machine node was hardcoded.
When we introduce the masked_gather node, we need to select instruction automatically,
in the standard way.
I added a flag "hasTwoExplicitDefs" that allows to handle 2 destination operands.

(Some code in the X86InstrFragmentsSIMD.td is commented out, just to split one big
patch in many small patches)

llvm-svn: 230471