[dev.simd] simd/_gen: parse SHA features from XED

[dev.simd] simd/_gen: parse SHA features from XED

To parse SHA feature instructions from XED, this CL added some utility
to decode fixed reg operands.

SHA512 parsing will be in next CL as we don't have SHA512 cpu features
in src/internal/cpu/cpu.go yet.

Change-Id: Id14cced57eab2ca9e75693a201f4ce7c04981587
Reviewed-on: https://go-review.googlesource.com/c/go/+/712181
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>