[llvm-mca][x86] Ensure avxvnni tests actually test the avxvnni instructions (#157892)
Noticed while checking #97271 - discovered we weren't actually testing
the vex variants of the vnni instructions in the avxvnni mca tests
Fixing this causes the znver4 results to break, because it turns out we
didn't have consistent instruction naming for the avx and avx512
variants, breaking the regex matching
So add the missing reg operand to the avx512 vnni instruction signatures
to match avx vnni