[NFC][Codegen][X86] Add a few more interleaved load/store patterns w/ i16 element type

Matching the costmodel coverage.
We want them both because they simplify coming up with the patterns
to check their cost, and to track their codegen.

Tests for loads can be fully autogenerated: https://godbolt.org/z/o1fncqo9n
For stores, however, i have done that semi-manually: https://godbolt.org/z/KPzTnvsh1