[X86] Add ISD node for masked version of CVTPS2PH.

The 128-bit input produces 64-bits of output and fills the upper 64-bits with 0. The mask only applies to the lower elements. But we can't represent this with a vselect like we normally do.

This also avoids the need to have a special X86ISD::SELECT when avx512bw isn't enabled since vselect v8i16 isn't legal there.

Fixes another instruction for PR34877.

llvm-svn: 350994