[X86] Redefine the 128-bit version of VPGATHERQD and VGATHERQPS to use a VK2...

[X86] Redefine the 128-bit version of VPGATHERQD and VGATHERQPS to use a VK2 mask instead of a VK4 mask.

This allows us to remove extra extend creation during lowering and more accurately reflects the semantics of the instruction.

While there add an extra output VT to X86 masked gather node to better match the isel pattern predicate. Currently we're exploiting the fact that the isel table doesn't count how many output results a node actually has if the result type of any can be inferred from the first result and the type constraints defined in tablegen. I think we might ultimately want to lower all MGATHER/MSCATTER to an X86ISD node with the extra mask result and stop relying on this hole in the isel checking.

llvm-svn: 318278