[dev.simd] cmd/compile, simd/_gen: make rewrite rules consistent on CPU Features

[dev.simd] cmd/compile, simd/_gen: make rewrite rules consistent on CPU Features

The previous CL left a bug in the xed parser so that the generator can
generate rules rewriting an AVX instruction to AVX512 instruction.

This CL fixes that.

Change-Id: I0df7e7dc6c936ce7add24a757ce7f44a15917fef
Reviewed-on: https://go-review.googlesource.com/c/go/+/703399
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>