[X86] Add more ISD nodes to handle masked versions of...

[X86] Add more ISD nodes to handle masked versions of VCVT(T)PD2DQZ128/VCVT(T)PD2UDQZ128 which only produce 2 result elements and zeroes the upper elements.

We can't represent this properly with vselect like we normally do. We also have to update the instruction definition to use a VK2WM mask instead of VK4WM to represent this.

Fixes another case from PR34877

llvm-svn: 351018