[dev.simd] cmd/compile: track which CPU features are in scope

[dev.simd] cmd/compile: track which CPU features are in scope

analysis for

- is this block only reached through feature checks?
- does the function signature imply AVX-something?
- is there an instruction in this block which implies AVX-something?

and keep track of which features those are.  Features =
AVX, AVX2, AVX512, etc.

Has a test.

Change-Id: I0b6f2e87d01ec587818db11cf71fac1e4d500650
Reviewed-on: https://go-review.googlesource.com/c/go/+/706337
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>