cmd/internal/obj/x86: add missing Yi8 for VEX ytabs

This change adds Yi8 forms for every ytab that had them before AVX-512 patch.
The rationale is backwards-compatibility.

EVEX forms remain strict and unchanged as they're not bound to any
backwards-compatibility issues.

Fixes #25510

Change-Id: Icd692266010ed64c9fe47cc837afc2edf2ad2d1d
Reviewed-on: https://go-review.googlesource.com/114136


Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>