[X86] Add support for forming vXi16 PMULH instructions from shifts.

We already form PMULH when the shift is truncated. But we can
also do it from just a shift by extending the result.

Unfortunately, I get regressions if I try to replace the truncate
combine with this as we turn the truncate into a more complicated
sequence first. Then we are unable to combine that sequence with
the extend produced at the end of this combine.

Differential Revision: https://reviews.llvm.org/D79682