[X86] Allow pmulh to be formed from narrow vXi16 vectors under...

[X86] Allow pmulh to be formed from narrow vXi16 vectors under -x86-experimental-vector-widening-legalization

Narrower vectors will be widened to 128 bits without changing the element size. And generic type legalization can already handle widening mulhu/mulhs.

Differential Revision: https://reviews.llvm.org/D54513

llvm-svn: 346879